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Tech Forum Questions

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

Introducing A Time Delay Into RC Car Steering 2019 Issue-2

A local STEAM program is looking into using an RC car or truck to simulate operating a vehicle on the moon or other astronomical object. One big opportunity to excel is introducing a time delay (say five seconds) into the steering system to model the time required for radio to go signals to and from the vehicle. The first thought is to buffer the command string at the transmitter or receiver. Ideas, hints, suggestions?

Jon Caples
Parker, CO

Reset System For Cable From Both Computer And Cell Phone 2019 Issue-3

My cable system needs frequent resets. That means turn the power off to the cable box (router) and to my own WiFi router, wait 30 seconds, and turn the power back on. This is really aggravating when I am in the bedroom or the garage shop trying to watch streaming video on my TVs there and the routers are at the other end of the house.

I want to build a WiFi based system so I can activate a control to cycle the AC or DC power from anywhere in the house either via one of my computers or my cell phone. Now, my phone is a smart phone, but not a name brand like Apple. It’s a Nokia and uses the Windows operating system, so the available apps are somewhat limited.

So, I’ll need an app that will work there and one that will work on my desktop computer running Windows 10. I would like some ideas on how to proceed. I would think that I could base this on an Ardunio or other single board computer which could easily handle a 30 second time-out.

I can figure out the interface to the power relay, some local PB controls, and a display. But how do I handle the controls through the WiFi? I need advise on a WiFi interface and the apps that are needed to activate it.

Edward Alciatore

Tuning The Music November/December 2018

Is there a way to vary the speed of a music CD player? I play my instruments along with the recordings and need to vary the pitch slightly to match the tuning of my instrument. We used to do this easily with vinyl by varying the speed of the turntable.

Klaus Herman
Raleigh, NC


CD players for professional/DJ market have variable pitch built in.

Erik von Seggern
Escondido, CA

I would be surprised. CD players are designed to KEEP their speed. I would convert the song to one of the usual audio file formats (mp3, wav) and run it on a computer with the free audacity tool. That lets you change the speed and even allows you keep the pitch! (Effect -> Change Tempo; Effect -> Change Speed). I use audacity constantly.

Werner G
via email

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

Good Timing November/December 2018

I have an old model automotive timing light that attaches between the #1 spark plug and wire. How can I make an inductive pickup to sense the signal so I don't have to detatch the wire? Is it even worth trying to

Ricardo Stewart
Hanover, MD

Proximity Detector November/December 2018

I want to build a backup proximity detector like many newer cars have, as a fun, useful project. The type that beeps in reverse when something is behind my car; faster beeping means the object is closer. I'm new to electronics, and I need help designing the circuit. I thought of using the PING))) sensors available from Parallax to sense the distance to objects. Any help with the circuit is welcome. It would be preferred to have it auto-activate when in reverse, but I could just as easily flip a switch mounted next to the shifter to activate if it becomes too complex.

Gregory O'Rourke
Newark, NJ

Audio Replay Device November/December 2018

Is there a device that will continuously "listen" to audio (via a mic or an earphone jack) and allow the user to push a button so that the device will repeat the last 20 seconds of what it just heard? I listen to scanners and ham radios, and often miss what was just said, and would like to get a repeat for clarity. I prefer a stand-alone device to an app or computer software.

John C
Montague, MI


A company by the name of Nuvoton makes a chip that records sound — voice if microphone connected — which can then be played back with the push of a button. The chip is ISD 1964 for 64 secs of recording in various lengths of time segments. This is a surface mount 28 pin SOIC package and can be soldered nicely with a fine tipped soldering iron. You can find the data sheets on Digikey.com. I also recommend Onstate Technologies (OnstateTech.com).

They make a good quality circuit board for the sound chip and has room for most of the needed other components (thru-hole) including a version with an audio power amplifier chip to drive a decent speaker. I believe that the chip can be operated directly from a few push buttons. (Record, stop, playback). It can also interface with a microcontroller.

Robert B.
Calgary, Canada

Reflow Oven Opinions Needed November/December 2018

Anybody doing SMT reflow soldering at home? Trying to make the switch from TH to SMT. I was wondering what homebrew oven setups people are using and any tips and advice for a newbie.

Joey Dampier
Reston, VA


Yes, I do SMT soldering using an electric skillet. Search the web for "solder skillet," there is a good write-up from the folks at SparkFun. Five to six minutes at 400 degrees Farenheit works great. Parchment paper under the board provides a way to lift it in and out of the skillet. Also, reasonably-priced hot air rework stations are now available, check CircuitSpecialists.

When I first started SMT, solder stencils were very expensive so I used the individual component stencils from Chip Quik. Some PCB houses now can provide stainless steel stencils along with your board for ~$30. For closely placed components you'll want a custom stencil.

Kerry Imming
Rochester, MN

A toaster oven can be used for reflow soldering work. I have one where one of the coils is broken, but the remaining coil gets it hot enough to melt the solder. I use an oven thermometer to monitor the temperature and approximately follow the recommended temperature/time profile.

Edward Alciatore
Beaumont, TX

I went down the path that you are, Joey. I was fortunate to have found SparkFun Electronics and even more fortunate that they had a kit with code ready to go. They’ve since discontinued that product, but there is a schematic and the code still on that site. I could simply regurgitate what is there, or not plagiarize by directing you there.

One thing that merits repeating is that you will need a toaster oven with at least 1500 watts (I used an Oster convection toaster oven) but 1800 watts would be better, especially if you’re going to use lead-free. That having been said, if I had this to do all over again, I’d not even use the timing aspect of this and simply experiment with how long it takes to melt the solder because it’is nearly impossible to replicate the Kester profile (that is published and out there if you look for it) with a toaster oven. Or, you can use a hot air gun (800-1000 degrees) to simply heat the board. It’s not as sexy, but it does the job just fine.

A word of caution on the toaster oven without using the timing function. DO NOT walk away, answer the phone, send a text, look out the window, chat with a friend... Do NOTHING except watch your work or you will have some very expensive, non-edible, toast.

Brendan Ames
Albuquerque, NM

It’s fairly easy and not expensive. People have successfully used toaster ovens, stoves, and kitchen hot plates. I use a $100 laboratory hot plate and monitor temperature with an infrared thermometer. The tricky part is laying down the paste for small SMT components. I strongly recommend an inexpensive stencil from oshstencils.com instead of doing it by hand.

Mike Hasselbeck
Albuquerque, NM

I worked for a while at a startup company where I (amongst other duties) built prototypes and very short runs, up to about 10 boards, that were nearly all surface mount technologies. If memory serves, at least one of the boards was about 8”x10” and had roughly 100 components on it, while others were a couple of inches on a side and only had a dozen or so components.

To build these boards, we would usually order a custom solder mask from Pololu (www.pololu.com) to use solder paste on the board. One word of caution is that solder paste has a relatively short shelf life, maybe 6 months, but that can be extended to at least 18 months if you keep it in the refrigerator. Take steps to make sure NOBODY tries to eat it!

Due to the nature of the solder paste, we wouldn’t start the process unless we knew we could finish it that day. We’d use the mask to put solder paste on just the spots we wanted. There are several techniques to hold the mask and board in alignment, then using good tweezers place all of the parts in their proper locations. Then we would “cook” them to “reflow” the solder.

Rather than using an oven, we used an electric griddle at that company. (DO NOT use the same one for cooking food later!) Once we had the boards ready to cook, we’d heat up the griddle using a non-contact infrared thermometer to check the temperature and for “hot spots.” We would use a flat metal spatula to transfer the boards onto the griddle, with something like a pair of tweezers or other small tool to slide the boards off the spatula and watch them closely to observe the solder becoming molten.

When all of the solder was molten, we’d wait another 5 to 10 seconds to make sure any hidden solder was also molten, then very carefully slide the boards back onto the spatula and transfer them to a heat-proof place where they could cool. I would usually only do one board at a time, though others sometimes would do two or three if they were small.

This technique works well for one or two of a kind boards that have parts that are manually “pick and placed.” A professional, time-temperature sloped oven is required for high-reliability boards, e.g., those going into space, but few hobbyists have this need.

I’ve never tried it with any ball-grid-array parts. We would send those out to assembly houses that also had x-ray inspection machines that could verify the BGA pins were all soldered.  I also found that it was best to leave off things like 100 lead quad flat-packs, and solder them on by hand. They would seem to invariably get jostled out of position and need to be removed and resoldered by hand anyway.

The larger and thinner the spatula the better. Don’t even think about trying it with any sort of plastic spatula — they’re all way too thick.

I also recommend that you practice on a board with only a dozen or so parts, none of which have more than 24 leads and accept the fact that you’re going to ruin a few before you get the hang of it.

Once you do get the hang of it, you’ll wonder why people are so scared of trying it — it’s really not all that hard!

Clark Jones
Gilbert, AZ

I converted a Black & Decker toaster oven into a reflow oven. Had to split off the cooling fan AC driver to a separate control line. I use leaded solder paste because of the lower melt temp & the eutectic parts to pads alignment. I only do design concept layouts & proof of concept boards.

Gail Schooley
via email

Logging Power Data November/December 2018

We have a Smart Meter installed for our home electric utility billing. Is there a way to read data from it to log usage data to a computer? I want to analyze my power usage by day of the week, time of day, devices and appliances in use, etc. Does the meter itself keep a log of this info that can be read or would I have to have a spreadsheet or other program to track it?

I'm not looking to interface any hardware to it. It would appear to have an RF link with the power company that might allow access. Any information about these meters would be helpful.

Lisa Miller
Cleburne, TX

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