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PA system to POTS interface 2019 Issue-4

I need to put the speaker-level output from a PA system on a POTS telephone line. I don’t need dialing capability, as I can connect a POTS phone to make the call, but once the call is established, I need a circuit that will:

  1. Hold the phone line open, as I’ll hang up the phone I used to make the call.
  2. Put the PA output onto the phone line, preferably with adjustable volume. I can attenuate the speaker-level PA output to line level if necessary.

If a reasonably-priced product with good sound quality (i.e., no hum, etc) is already available to do this, suggestions are welcome. I’ve found many products for recording phone conversations or putting them into a PA, but only quite expensive ones with way more features than I need that will do the simple job I want.

Thanks for any help/suggestions.

Gary Rathbun
Placerville, CA

Image Reverse 2019 Issue-3

I’m building a rear facing camera system for my car that displays the image on a 7” screen mounted on the dash. However, the image appears reversed horizontally. Is there a circuit to flip the image left to right so it would be correct from the perspective of the driver? Any ideas are appreciated.

Krystian Czarnecki
Lombard, IL


Your best bet is to buy a backup camera that reverses the image for you. Many models let you select which way you want the image displayed. Depending on the resolution and night vision features you want, the backup camera can be purchased for under $20 online.


You don't need any electronics. Instead of having the camera face backwards out your rear mirror, place it so it "looks" left or right. Then use a mirror at 45 degrees to the camera's optical axis (top view) and you'll get a left-to-right reversal of what your display shows. You could mount the camera and mirror on a plastic or plywood base and support the mirror with a small angle bracket that lets you adjust the angle so it suits your purpose. And you can move the base as necessary.

Jon Titus
Herriman, UT

Coil Winding Tips Answered 2019 Issue-1

I’m looking for some pointers on winding coils. On a second layer of winding, should the direction reverse when reaching the end of the core, or return to the starting side and wind in the same direction? It seems like winding in the reverse direction would cancel out the field. Does it matter which direction?

Jordan Bracegirdle
Toronto, CN


Assuming that the coil is being wound on a bobbin or other form, the coil is wound beginning at one end, laying down the wire turn by turn until the opposite end of the bobbin is reached.  At this point, winding continues in the same direction, but proceeding turn by turn back to the point of beginning, and so on. For an inductor, it doesn't matter which way you wind the turns so long as you're consistent throughout.

Others have raised this question; see, for example, https://electronics.stackexchange.com/questions/369884/how-to-wind-an-inductor.

A good document on building inductors was found on the ARRL site:


Good luck.

Peter A. Goodwin
Rockport, MA

It's the direction the current flow rotates around the former that matters. This does not change when your winding goes back along the core.

Kit Wareham-Norfolk
Exmouth, Western Australia

It is OK to wind a bobin from one side to the other and back. That is the way it is normally done. If you were to go back to the same side every time, you would end up with a nasty bump in the coil layers where the wire returns. And no, that does not cancel the flux field. Looking from the end of the coil, the windings are still in the same direction, either clockwise, or counter clockwise. If you were to change direction, say part of the coil is clockwise, and you now reverse that direction to counter clockwise on the next part, than those windings would cancel out the same number of earlier windings (in an idealized scenario). Those windings would become a simple resistor from an electrical perspective.

Bill van Dijk

Troubleshooting Circuit Needed 2019 Issue-2

I’m looking for a circuit using a piezo buzzer, that I can hear from the room above the cellar. It will be connected to a Taco Heating Zone Valve on the boiler in the cellar for troubleshooting the thermostat when it goes on and off.

G Forcino
via email


I’m looking for a circuit using a piezo buzzer, that I can hear from the room above the cellar. It will be connected to a Taco Heating Zone Valve on the boiler in the cellar for troubleshooting the thermostat when it goes on and off.

Get a DC-DC converter like this one:

DC-DC Buck Step-down Regulator Converter 4.5~65v to 3V 6V 9V 12V 24V 48V Module

Get a diode, resistor and capacitor per the schematic and build the new circuit. Note the capacitor must be rated at least 35 volts. Connect the new circuit to the existing circuit, A to A and B to B.

The diagram shows a doorbell push button - more on that later. In place of the doorbell push button you can connect whatever piezo you want that is within the capability of the DC-DC converter you get. The problem with piezo buzzers is that it is impossible to predict whether you will hear them from the room above.  A cheap (<$10.00) solution is to use a wireless doorbell. For that, read on.

Get a wireless doorbell from eBay like this one: https://www.ebay.com/itm/400ft-Wireless-Doorbell-Twin-Wall-Plug-in-Waterproof-Cordless-Door-Chime-Kit/152986512099?hash=item239eb4eae3%3Am%3Amxl6HSV0iPKKQ83VnZkaAfw&LH_BIN=1

Set the output voltage of the DC-DC converter to match the battery voltage of doorbell push button circuit. The battery will not be used. Short the switch contacts inside the doorbell push button.

When the thermostat sends 24 VAC to the Taco, the DC-DC converter will output whatever voltage it is set to to make the wireless doorbell chime or the piezo buzz. When the thermostat is open, there will be no voltage out of the converter.

via email

SCSI Card Or Adapter Needed 2019 Issue-1

I have a bunch of old optical disks that have data archived from about 20 years ago that I would like to access. I found the stand-alone drive in a box of old computer junk, complete with the big SCSI cable. Problem is that I don’t have the computer anymore that has the SCSI card to plug it into, and don’t even know where to begin to look for one.

Is there something comparable that will work with a modern computer? An SCSI to USB adapter maybe? What about drivers? The old machine was probably Windows 98.

Mark Cisneros
Columbia, TN


I have a Win98 computer set up to read SCSI disc drives. It was a bear to create and find drivers for. I’m in Phoenix Arizona.

My next question is what software would be needed to read the data, is it just text? My setup was crated to be able to read and copy SCSI to sim cards, because of the lack of production of SCSI drives. I can currently copy from SCSI to sim cards if that would help. SCSI is old technology that is not really supported. If the data is important, I could probably read your drives and forward the data back in a usable format.

Let me know.

Antoinette Sides
via email

Adaptec made a USB 1.1 and a USB 2.0 SCSI to USB adapters; USB 1.1 M/N FX0C21902KL P/N 1861400; USB 2.0 M/N FX0A229005U P/N 1989100. I have one of each and they worked on every device I tried them on. Obviously the USB 2.0 is faster. I think the main drivers were Windows 98, but I can't recall for sure. I think I used them last under Windows XP.

Alexander Fisher
Galloway, OH

There are still PCI cards available for your SCSI drive, although they go by either GPIB or IEEE488 interface. National Instrument has both the cards and drivers (http://www.ni.com/en-us/shop/select/gpib-instrument-control-device). It would be difficult to connect the SCSI drive to the computer with a USB connection since the drive requires several control lines for proper access.

Lance Corey
Santa Ana, CA

I find some SCSI/USB adapters on eBay for around $170 to $300 each. I also find brand new SCSI/PCIe host adapters on Amazon for under $100. Either way, unless your cable is a Macintosh style cable (DB/25 to AMP50), you’ll also need a new cable. (For most of the PCIe host adapters, you’ll still need a new cable). What make and model are the optical disks? You may be better suited to pick up a used MO drive that’s SATA and use it on your current PC. Or with a SATA/USB adapter externally.

Ralph Phillips
Bossier City, LA

Ringing Of The Bells Answered 2019 Issue-1

Is it possible to replicate tubular bell tones with a microcontroller? I’m needing realistic, deep, resonating tones. If so, are there any micro requirements that would make one device more suited over others and how do I create the tones? I want to program short 10-15 sec jingles of my own composition.

Sara Hanchett
Forest Grove, OR


For the best result I would look at a MIDI solution with a sampled sound file.

Bill van Dijk

Devices/Software/Scheme For Wi-Fi Remote Control Using Smartphone 2019 Issue-2

There are a number of articles in the past issues of Nuts & Volts for a variety of ways to control devices in a home using a variety of control devices and schemes. However, none seem to have the combination of characteristics that I want. I would like to control a pair of simple low current AC sockets (ON/OFF) from my cell phone. I would like to use a combination of an existing app, a PIC style device (Ardunio or whatever), and a Wi-Fi module to connect it to my existing Wi-Fi. I do not see why a $100 hub should be needed. My present cell phone uses the Windows operating system, but I know that apps for it are scarce and I am willing to get an Android device. I’m stuck on finding the cell phone app and the Wi-Fi module. I would assume that I would have to write the appropriate code for the PIC device. I can interface the PIC with a relay, probably a solid state one. Any suggestions?

Edward Alciatore


If all you want to do is to be able to remotely control an AC outlet via a mobile device from anywhere, then simply buy a WEMO Mini Smart Plug. These are sold in Home Depot for about $25. They connect to your home WIFI, are very easy to install and have APP’s for both Android and Apple IOS devices.

These Smart Plugs have a switched AC outlet capable of carrying up to a 15A load. No code to write, nothing to build, just download the APP, plug the WEMO Smart plug into any AC outlet, follow the APP instructions and you’re up and running in five minutes. Install as many devices as you need remotely switched outlets.

Roger Baker
Redmond, WA

The easiest way to do this would be to buy some TP-Link KASA modules. No hub required — just plug them in, load the app and in a couple of minutes you’re done. AND, they work with Alexa/Google Home.

Bruce Robin
Naples, FL

Sonoff makes a bunch of home automation wifi devices. I have several of their products which I got off Ebay and AliExpress. Just type “Sonoff” on the search line. They do a lot more than just switch devices on or off. I paid about $6 for a single channel and about $15 for a 4 channel. Rated 10A @120VAC.

Simple to setup, the device just needs to be within range of your router. The free app is only available for Andriod/Apple phone, although they say you can use a PC if you run it thru an “Android Emulator” program. Never tried this though.

Rochester, NY

DC To AC 2019 Issue-1

I have a 100 amp 110 volt DC generator. I want to convert the output to 60-cycle AC. Does anyone have a schematic or info to build a converter? How much power loss can I expect in the final converted output? Thanks for any info.

Lucio Saunders
Indianapolis, IN


You don’t need a converter. You need an inverter. A converter is used to change AC into DC or DC of another voltage, while an inverter changes DC to AC. That being said, there are many ways to skin this cat, depending on how much you want to spend and what you want to use the AC output for. Before we go there though, we have to start with basics: the generator.

In your question, you did not specify the type of DC generator that you have or how you are driving it. There are three types: the shunt; the series; and the compound. Each has its own set of operating characteristics. I assumed that yours is of the shunt type, the most common. The Rheostat (R) is used to adjust the output. The voltage (E) and current (I) are monitored and the output is supplied through a 125 amp fuse.

Your generator is rated for 100 amps at 110 volts which is 11,000 watts (11,000/750 = 14.67 horsepower). Add 20% because no generator is 100% efficient, so about 18 horsepower is required to drive it at its rated speed. This is important because the output of a DC generator is related to both its field strength and operating speed.

There are three ways to do what you want to do.

The first option is the simplest — just use a 110 volt DC to AC inverter. These are around because they were once used on small hydro electric systems. You may be able to find one on the Internet, but I don’t recall any being large enough to handle 11,000 watts.

The second option is a bit more radical, but much more feasible as the inverters are widely available for usage in the alternative energy field. A pair of 48 volt magnum energy MS4448PAE inverters are connected in series and these are in parallel with a pair of series connected high-value capacitors. An equalizer line runs from the junction of the capacitors to the junction of the inverters. This works because these inverters like to operate around 55 volts and because their outputs are in parallel, they share the load evenly. The equalizer line is a safety to handle any imbalance.

As for output, these inverters are rated for 4,400 watts continuously, 4,800 watts for 30 minutes, and 6,000 watts for 30 seconds. The first two specifications are all that matter in your application. Assuming 90% inverter efficiency, the generator will supply 9,680 watts (88 amps) to produce 8,800 watts of AC. The generator will supply 10,560 watts (96 amps) to produce 9,600 watts of AC for 30 minutes at a time.

The third option is the most radical, but it allows two things the other options do not: redundancy and the ability to supply AC to the utility grid. This option utilizes microcontrollers commonly used on photovoltaic modules to convert low voltage DC into 60 Hz and then to synchronize this to the utility grid. I have chosen the Enphase Energy 250 watt unit. You will need 36 of them and they are used in groups of three to allow an input of 48 to 144 volts. This is well within the range of your generator.

The outputs of all the units can be connected together to produce about 9,000 watts of AC power at 120/240 volts or they may be connected for three-phase output at 208/120 volts. They must be connected to the AC grid to operate though. By having 36 units in 12 series strings, the failure of a single unit will only reduce the output by a little more than 8%. It was nice talking about the good ole days of DC. If you have any other questions, you can contact me directly at PO Box 9106, Concord, MA 01741.

Craig Shippee
Concord, MA

Side Lights As Turn Signals - MG Auto 2019 Issue-2

My brother has an old MG that he likes tinkering with. He wants to use the side lights to indicate when the turn signal is on for drivers that are on either side of him. Normally, the side lights turn on and off with the headlights.

In his case — and to reduce peak power consumption to the blinker circuit — he would like the side lights to turn on when the signals are off, and vice versa (when the turn signal switch is turned on). Additionally, if the lights are on, the side lights should give priority to the turn signal, turning on and off, opposite to the signals in the front and rear, and returning to full on when the turn signal is not engaged.

I think the simplest way to do this would be with solid-state relays and the use of some logic gates (for each side). I have built hundreds of logic circuits, but have not really dabbled much in automotive applications. I know the electronics have their own temperature and performance specs and are noisy environments, not to mention the notoriously bad reputation that English cars have for electrics. Can you recommend a circuit for this application? (This car is a NEGATIVE chassis).

Patrick Gilmore

About Resistors November/December 2018

Could someone explain what pull-up and pull-down resistors are, when and how they’re used, and how to calculate their values?

Andy Dietrich
Dallas, TX


Resistors are used in a number of roles in circuits. The terms “pull up” and “pull down” are used to describe two of those roles. A somewhat precise statement of the definition of a pull up resistor would be that it is a resistor where one side/lead is connected to a circuit device, often a transistor or FET or other active device, and the other side/lead is connected to the positive power supply voltage used for that circuit. Thus, it is trying to “pull” the terminal of that device “up” to the level of that power voltage. A “pull down” resistor would be the same idea but with the negative power supply voltage or ground in place of the positive one.

Active devices, like transistors, can be modeled as a variable resistance that is controlled by the base current: the greater the base current, the lower is that effective resistance (emitter to collector). So more base current equals more current in the emitter-collector circuit. But that, by itself, is not very useful in most circuits where an amplification of the voltage is what is needed. By adding a “pull up” or “pull down” resistor, depending on the polarity of the transistor and the arrangement of the circuit, a voltage divider is created. That voltage divider consists of the fixed resistance of the “pull up” or “pull down” resistor in series with the variable resistance of the emitter-collector path through the transistor. The output voltage is taken at the junction of the resistor and the transistor. That is where the variable current is converted to an amplified voltage.

The idea is pretty much the same with FETs and other active devices. “Pull up” and “pull down” resistors are also used with switches. The resistance of the switch is also variable: either zero or infinite. In that case the resistor will pull the output of the switch all the way to either the positive supply voltage or to ground.

The values of the “pull up” and “pull down” resistors are usually calculated with Ohms Law. The circuit designer must have a value for the current that will flow in the resistor/transistor circuit. There will be several concerns for that value including the load impedance, the current rating of the transistor, the desired point of operation on the characteristic curves of the transistor, the amount of quescient current that is allowable/desirable, and others. Once that current value is determined, it is combined with the supply voltage in Ohms Law to find the value of the “pull up” or “pull down” resistor. R = E/I

If the current is significant a calculation of the power dissipation in the resistor should also be made: P = VI where V is the supply Voltage and I is the maximum current that can flow in the resistor. A general rule is to use a resistor that is rated at two times the power level that this calculation produces as a safety margin. Often (usually) the resistance value of the “pull up” or “pull down” resistor is made as high as possible in order to avoid the use of high power resistors.

Edward Alciatore
Beaumont, TX

Pull-up or down resistors does not refer to a type of resistor, but its function. Usually their purpose is to give a high impedance wire a known state. For instance, an output pin on a MPU may be an open collector output, capable of sinking a few mA. When it is activated, its state is grounded. When released, it floats, and has no definable state. In this case a pull-up resistor connected to the supply line would give it now a high level.

Pull-ups and pull-down resistors can be used in many locations, another example would be in an I2C buss. Read up on that, most descriptions will do a good job of explaining how it works.

Bill van Dijk

The basic idea of a pull up or pull down resistor came from some logic families which had output pins connected to the collector of the output transistor stage. There was no internal connection to a power supply, so the output did not respond to logic states if no other connection was made. A suitable resistor was tied to the plus supply, so when the base of the transistor was held low the output went to the supply voltage (pulled up to the supply), and when the base of the transistor went high it would pull down the output to ground. The open collector allowed outputs to be wire ORed together saving parts. By connect several outputs to the same resistor, if any of them them went low it would pull the resistor down (an OR situation).

Values for these are usually suggested in the data sheets for the parts. To calculate a value get the max output current from the data sheet and use your supply voltage and ohms law to compute the smallest allowed resistor.

The idea has been extended to resistor networks which aid in driving high speed signals through long lines. A google on impedance matching and line driving will get you started on this variant.

Warren O Wilderson
Eagle Point

Pull-up and pull-down resistors once baffled me, too. Very often a device, say a 555 timer, will require a ground pulse to start a timing cycle. This means that one might install a momentary switch to ground to activate the timer. Well, if that pin on the chip is not already in some known state, it can behave differently each time it’s powered up. It may work as designed, it may not. If the wind blows from another direction, it may behave differently because it is “floating.”

Now, you could just connect it to the voltage that’s supplying the chip, but if you then forced that to ground, you’d have a short circuit and something is going to burn. If you have something held to supply voltage, say 12 volts, by way of a 10k resistor, it will sit there seeing 12 volts. If you then press the momentary button, the subject pin will see it as a ground and the 12 volts will cause 1.2 mA to flow momentarily.

So, you don’t have a short, the activating pin sees a ground pulse and starts working, and you won’t damage your circuit. Leaving any pin, even ones that specify “NC” (no connection) floating, may yield unexpected behaviors and not always the same “unexpected behavior.” In this case, it’s good practice to ground pins labeled “NC” either directly or by way of a “pull-down” resistor. If you always use a resistor, you minimize the chance of a short circuit even if you made a mistake in grounding the wrong pin. Hope this helps!

Brendan Ames
Albuquerque, NM

A pullup/pulldown resistor resolves a voltage ambiguity that may exist on a circuit node. These resistors are commonly used with transistors in digital switching operations.

A pullup resistor connects a node to logic 1 voltage (eg 5V) and a pulldown connects to logic zero (ground). For example, a bipolar transitor used for digital switching has a pullup resistor connecting logic 1 voltage to the collector node, i.e. the transistor switch output. When the transistor is off, no current flows through it or the pullup resistor. Ohm’s Law shows there can be no voltage drop across the resistor, hence the voltage on the collector must be at logic 1. When the transitor is on, current flows through the collector and pullup resistor. A voltage drop now exists across the resistor causing the collector voltage to drop below the level of logic state 1. The more current that flows, the bigger the voltage drop.

When a component is specified as open collector, the designer must supply the pullup resistor. This is the case for the I2C communication interface. The designer may choose a large pullup resistor to limit current and extend battery life. But there is a trade-off due to unavoidable stray capacitance (C) at the output. A large resistor will increase the RC time constant and slow down the response.

Mike Hasselbeck
Albuquerque, NM

Pull-up and Pull-down resistors are used to force an input signal line to a default level when there’s no external input applied. Typical pull-up values for Digital lines are between 1Kohm and 10Kohm, connected between the V+ line (i.e., +5 VDC) and the signal line. Pull-down resistors do the same thing, except they’re connected between V- (or GND) and the signal line, and are usually no smaller than 4K ohms.

To calculate the resistor size, you need to know how much current is going to flow INTO the line (pull-up) or flow OUT OF the line (pull-down). You typically don’t want more than 20 milliamps of current flowing through a pull-up nor do you want more than a few milliamps flowing through a pull-down. For +5V logic, pull-ups from 1K to 4.7K are sufficient.  You rarely see pull downs with +5V logic, but they are needed sometimes.  The power rating of the resistor is calculated using the (I^2) x R, and rarely goes above ½ watt.

You can connect a pull-up and pull-down in series, with the resistor junction connected to the signal line to form what’s called a receiver. You’ll typically find these on serial communication lines where the SIGNAL LOW value is a negative voltage. The pull-down resistor will usually be sized with the pull-up to create a voltage divider. You can find dividers with CMOS and +3.3V logic where you don’t want the HI to go above a certain value nor the LOW to go below 0V (GND).

Here is a good reference: http://www.resistorguide.com/pull-up-resistor_pull-down-resistor/.

Ken Simmons
Auburn, WA

To begin with, positive voltages are considered ‘up’ and negative voltages are ‘down’. A resistor connected to a positive voltage on one end and to a circuit element on the other end will be pulling the circuit element up to the positive voltage, if there is no current. If you want current to flow, you must figure how much and compute the resistance from: R=V/I; where V is the voltage drop across the resistor. Pullup resistors are often used with an IC that has an open collector output.

Pulldown resistors are most often used on the emitter of an NPN transistor. If the base of the transistor has a positive voltage applied, the negative voltage could be zero (ground). In any case, you need to know something about the circuit to compute the resistor value.

Russell Kincaid
Milford, NH

For a good discussion of pull-up and pull-down resistors, visit: https://www.electronics-tutorials.ws/logic/pull-up-resistor.html.

Jon Titus
Herriman, UT

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