I’m looking to purchase a belt grinder for my workshop to smooth the edge of circuit boards, aluminum cutouts, and the output of my 3D printer. My question relates to the variable voltage controllers and three-phase motors that are available as optional equipment. Given almost double the price of the grinder, can I simply use a surplus variac on a single-phase motor to get a variable speed grinder?
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The short answer is no, you can’t use a variac on an induction motor.
The speed of an induction motor is controlled primarily by the line frequency, not the voltage. If you try to limit the voltage with a variac the motor will likely overheat as it draws additional current to try to compensate for the lower voltage.
What might work for you is a universal (brushed) motor. They are called universal because they can run on either DC or AC. This is the type of motor that is used in home treadmills, handheld routers, jobsite circular saws, etc. The speed can be controlled with a SCR (variable phase) speed control. These motors are usually rated for 120V single phase, up to about 15 amps or so. You could conceivably use a variac to control one of these but the SCR or Triac variable phase speed control will provide better low speed performance.
The speed of an induction motor is dependent primarily on the frequency of the alternating current driving it. Speed controls for 3-phase induction motors are often called VFDs which stands for Variable Frequency Drive. They work by converting the incoming AC power to DC and then use a 3 phase inverter to convert it back into AC with the ability to vary the frequency from as low as a few Hz to 120Hz or more for motors that can tolerate the increased speed.
If you attempt to reduce the speed of an induction motor by controlling the voltage with a Variac, you will likely overheat the motor without it slowing down much, because the current will increase as you reduce the voltage. Inexpensive variable speed power tools are powered by universal motors that have brushes and a commutator and these can be controlled with a Variac, though the common triac light dimmer type circuit is far more economical.
If you can find a grinder powered by one of these then your idea will work, but most bench mounted tools use induction motors.
Also note that the nominally fixed speed motors of any type usually contain an integral cooling fan under a cover, mounted on the shaft. These are good enough, but if you run the motor slower for a significant time, you could fatally overheat the motor.
If it gets hot, add a separate cooling fan so the motor can take It!
I have an older pair of Cerwin-Vega speakers where the foam edge has rotted and fallen apart on the woofers. I would love advice on the pros and cons of “re-coning” vs. buying new woofers. The model of the speaker is “R-24.”
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A quick Google search for “speaker reconing” will result in 100’s if not 1000’s of hits for this service or parts. Many offer a DIY kit while others are businesses offering the service or an exchange program for speakers.
We know of one business in our area that does the reconing/refoaming service:
TV Service in Crete, Nebraska. His webpage is www.reconespecialists.com
Phone: 402-826-3540
My personal opinion, I would enlist the services of someone experienced unless you really feel confident in doing the work yourself.
If the cone on your Cerwin- Vega speaker is still good, why not consider purchasing a surround kit. They are available online and maybe at your local electronics parts store. Go online and look up your particular model of speaker’s surround kit. They are usually priced around $20-$30 depending on the make and model. It’s an evenings work, but if you have some manual dexterity, with some care, you can have your speakers back up and running like new.
I have been successful doing this on several JBL, Altec, and Bose speakers. I suggest that you stick with U.S. made surrounds. Follow the instructions, and make sure to use a low frequency signal from either a signal/function generator/oscillator or the CD that comes with some of the kits to make sure that everything is centered, and that the voice coil doesn’t rub on the pole piece, etc. Good Luck!
I was recently given an old pair of Cerwin Vega speakers from a co-worker in much the same condition, it sounds, as yours are. The woofers in both cabinets were electrically good as was the physical condition of the voice coils and cones. Both woofers though, exhibited the common problem of foam rot. Not only does the foam separate but seems to dissolve leaving the cone and voice coil to rattle around without direction.
I performed a web search on the subject and found a solution that seemed very reasonable. Simply Speakers (simplyspeakers.com) has re-foam kits for numerous speaker manufacturers including Cerwin Vega. I’m not touting Simply Speakers as the only provider, but they were excellent to work with, sent me everything I needed to do the job and provided a youtube video on how to do it. Search youtube for “Simply Speakers” to see the video. I just finished mine last week with excellent results.
If the cone and the driver are in good condition, I would do it. It isn’t very expensive or difficult. I replaced the flexible foam on a set of old Speakerlab speakers about 5 years ago. There are a number of places on the internet and on ebay that sell the parts and they have instructions. I’d check out some of the sites and look at the instructions to see if you feel comfortable with the procedure.
For the subwoofer, I had to repair the foam on both the 10” speaker and a passive radiator. I did decide to make a ring out of 1/4” plywood to make the assembly of one of the larger rings easier but you probably wouldn’t need to do that.
I’m building a geocache container. To make it look authentic, I’d like the red light to flash every few seconds. Unfortunately, I don’t have any “Nuts & Volts” electronics skills — what type of long-lasting battery and low-power LED light to use — and am hoping to get some expert advice.
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Search online for “firefly circuit 7555” and you will find what you want. (Or “unijunction transistor flasher” for the old fashioned method). The LED should be a high brightness type with diffused lens, at least 300mcd. The LED’s above 1CD typically use clear lens, so can’t be seen off axis. As an alternate, if you only need it to run a couple of days, then just solder a L-36BSRD flashing LED inside a 9V battery snap, I make these up for cavers, they leave them behind like breadcrumbs to find their way back.
I have satellite radio (Sirius) aux audio output connected to DLO TransPod aux input. The TransPod is normally used for iPod to FM radio in a car. I am using it in my house.
The audio is transmitted on a frequency to my FM radio (50 ft away) with no power to the TransPod! How is this possible?
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It appears the output of the Sirius unit is supplying “phantom power” to the TransPod. IOW, there is a small DC voltage (1-2 VDC) present on the output signal that the TransPod is using as its’ power source. In a way, it is “magic” as phantom power schemes are regularly used in audio PA systems to supply power to electret microphones without using external batteries.
I am trying to build a small micro ampere meter project I found on the Internet (See schematic). The design calls for a +9V and -9V supply. Is it possible to modify this circuit to use a single 9V battery instead of two?
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That’s a very old circuit and it has at least one glaring flaw: The value of R3 is way too high. I suspect it should be 3.3K instead. Also, D3 and D4 seem unnecessary because there can never be more than about 2mA of meter current. R1 protects D1 and D2, but microamp circuit levels wouldn’t threaten them. The input bias current of a 741 op-amp through a 10K resistor produces up to 5mV offset that’s not temperature-compensated. It’s nullable, of course, but the null can drift.
Figure 1 |
While it’s pretty easy to make a DC voltage inverter with circuits like the one in figure 1, I believe there’s a better solution: Single-supply op-amps are available with offset null pins, completely eliminating the negative power supply! See figure 2 for my (untested) circuit suggestion. TLE2021 chips are available in plastic DIP packaging from the usual suspects, such as Digi-Key and Mouser.
Figure 2 |
The current sampling resistor should be selected for full-scale reading with 10mV drop, which is 10X lower measurement burden relative to the original circuit.
If you add the input diode protection resistor back in, then you incur only 0.7mV max uncompensated offset because of the TLE2021’s enhanced performance. And, if you do this, then there’s really no reason to keep those diodes -- you can rely on the op-amp’s input pin protection circuit.
My flat panel TV doesn’t have a headphone jack. It has dual RCA jacks labeled “R/L line out” and a “TOSLINK” connector. What would be the simplest way to get my headphones hooked up to this TV?
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Chad, a small headphone amplifier could be used. Look at the ones for amps and guitar accessories.
The easiest way to listen to your TV via headphones is by using a set of computer speakers connected to your TV using a stereo 1/8” (3.5mm) jack to RCA plugs y-adapter (see below).
Plug:
Turn the computer speaker on and adjust the volume to your liking.
Here are some sources for the y-adapter:
Parts Express
www.parts-express.com/dual-rca-male-to-35mm-stereo-female-y-adapter-audio-cable--240-070
MCM Electronics
www.mcmelectronics.com/product/24-13885
RadioShack
NOTE: RadioShack doesn’t have a “direct” adapter, you need to create one from the following parts - http://comingsoon.radioshack.com/3-ft-1-8-stereo-to-dual-phono-rca-plug-y-cable/4200494.html#.VaesjHWuPIo and http://comingsoon.radioshack.com/1-8-stereo-phone-coupler/2741555.html#.VaesxHWuPIo
Finally, depending on how far away from your TV you are, you may need the appropriate length male-female stereo extension cord to patch the adapter to the computer speakers.
I would use a external computer speaker set. It will provide a cheap small amplifier, volume control and an input for your headphones to plug into. If it comes with an external power supply you won’t need to buy batteries. The computer speaker system will most likely come with a stereo mini 1/8” phono plug so you will also need to get an RCA adapter cable. The computer speakers must have a place for you to plug in your headphones.
Attach the adapter to your TV and then plug the speakers into the adapter. Once the speakers are connected you should be able to play audio out of them. Then plug in your headphones and the speakers will shut off and all the sound will feed into your headphones. Adjust the volume on the speakers.
Done and your total cost should be less than $25 (15-$20 speakers and $5 for the adapter).
What’s the difference circuit-wise between a “dimmable” and a “non-dimmable” 110V home LED light bulb?
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The non-dimmable variety use a largish electrolytic capacitor in the power supply, to generate a DC voltage, and then the LEDs are driven with a constant current circuit. So because of the constant current, the LED output is unaffected by supply voltage, so the brightness won't change with voltage(or dimming). Dimmable leds only have a small film capacitor, and the control circuit sets the current according to the input voltage (so they pulsate at mains frequency) and when dimmed the light output is chopped up similar to an incandescent bulb. The above applies to screw in bulbs, larger LED supplies may still use electrolytic capacitors, but within these another circuit measures the incoming duty cycle and adjusts the LED current to match.
Does anyone have a circuit for a homemade “tie breaker” system? A teacher at my son’s school is having a quiz contest where kids have to “buzz in.” I need to build a circuit that can indicate who pressed a button first. I would prefer to use simple electrical components for this project as I am not really adept at programming microcontrollers.
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The tiebreaker circuit could also be implemented with relays, which wouldn't require any soldering or breadboarding. A relay would be required for each contestant, with as many poles per relay as there are contestants. The schematic I provided uses four 4-pole relays. For a higher number of contestants, relays could be ganged together (wire the coils in parallel) to obtain more poles. For each relay, pushing the corresponding button activates the coil; the normally-closed contacts of the other relays in series ensure that only one coil can be activated at once. A normally-open contact of the same relay placed in parallel with the button latches the coil so that it will stay on when the button is released. A normally-closed reset button (which could also be the on/off switch) will unlatch the relays.
Low cost multi-pole relays with screw-down wire terminal sockets can be found on eBay or at electronics surplus stores. Some even have indicator lights which would eliminate the need for separate lamps. Supply voltage would be selected to match the relay coil voltage. Since relays are mechanical devices, they have a finite switching time, which is usually on the order of milliseconds. It is theoretically possible that two button presses extremely close together could cause more than one relay to latch. To test this I put together a system like I described but with only two relays and buttons instead of four. The good news is that I was unable to make both relays latch on, even after many attempts at pushing both buttons simultaneously.
This is one of the simplest quiz circuits I've come across. I've built several versions. Some as simple as the first one, and a very complex one, but still using the same concept of parallel SCRs. www.techlib.com/electronics/games.html
There is a very simple old school tie breaker indicator circuit which I have used several times, It's inexpensive and works well.
Use a small neon lamp such as an NE2, and NE51 or any of the other types available. Use as many lamps as necessary for the number of stations desired. A latching type pushbutton or toggle switch in series with each lamp located at each players position. The trick here is that all of the series lamp/switch combinations are paralleled from one power source that being the 120 VAC line with a 68k-100k 1/2 watt resistor in series with one side of the line.
The principle is simple, with all of the switches off, there is no drop across the resistor. The first lamp that is energized, fires and pulls the voltage on the bus down to the point where no other lamp can fire. Neon lamps need a high voltage (generally around 65 volts) to fire but once fired will stay illuminated on much lower voltage, lower than any other lamp can fire.
The attached circuits (Figures 1 and 2) should do the trick. Each student station is equipped with a normally-open pushbutton switch connected to the instructor’s console via a cable of suitable length, terminating at a connector.
The instructor’s console consists of a bank of LEDs, one per student; a Master Reset pushbutton switch; and a suitable number of two-pin connectors into which the student pushbutton cables are plugged.
Each student readout consists of a “D” flipflop, an AND gate, an inverter, a diode, and an LED, plus associated wires, resistors, and capacitors as shown. The LED can be red or green, of any physical shape, having a maximum continuous current rating of 20 mA. The circuit is set for about 10 mA through the LED, which is very conservative and will provide quite adequate illumination.
The CMOS 4013B contains two “D” flipflops as shown, in a 14-pin package; power pins are 14(+) and 7(-). The CMOS 4081B contains four two-input AND gates as shown, in a 14-pin package; power pins are 14(+) and 7(-). The CMOS 4049B contains six inverters as shown, in a 16-pin package; power pins are 1(+) and 8(-). Be careful of this — applying “+” power to pin 1 is unconventional, but this is the way that the package is designed.
The circuit works in the following manner:
This ensures two things:
The two resistors shown relative to the output and one input of each AND gate provides hysteresis and reduces noise sensitivity. As implemented, the input signal delivered to the 12K ohm resistor must rise above eight volts or fall below 4.5 volts before a high or low output level (respectively) will be realized at the gate output (assuming that the remaining input to the AND gate is held high).
The contacts in each pushbutton are debounced with a paralleled R-C network.
The 12 VDC power can be provided by a small wallwart regulated power supply. Anything that will provide 100 mA or more will suffice.
Construction Suggestions:
Datasheets for the three ICs may be found at:
www.nxp.com/documents/data_sheet/HEF4013B.pdf
www.nxp.com/documents/data_sheet/HEF4049B.pdf
www.nxp.com/documents/data_sheet/HEF4081B.pdf
A 1N914 diode datasheet is located at: www.vishay.com/docs/85622/1n914.pdf
Parts are all readily accessible from Digi-Key, Mouser, Jones Associates, Newark, etc.
I have modified a circuit that I used 27 years ago, which consists of a two input NAND (cd4011n) driving a set-reset flip-flop (cd4013n). The push button switch is connected to one 4011 input and goes high when pressed. The other input of the 4011 is held high by a resistor to VCC.
When the button is pressed, Q of the 4013 goes high, lighting the LED. At the same time, the NOT-Q output goes low and locks out all 4011 inputs including the one that pressed the button. The circuit remains in this state until the reset button is pressed.
My home alarm system has a motion sensor that has failed and the alarm company wants $89 for a new one! I removed the bad one and it has screw terminals labeled:
The simple PIRs I find for use with the Arduino are 5V and they don’t have “tamp” pins. Can someone provide a schematic on how to hook up one of these low cost replacements?
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There are several reasons an Arduino hobby type PIR is not a good substitute for a commercial PIR motion sensor.
The $89 quoted by your alarm company sounds fair, and is an even better deal if they install it for that price. If you are under contract for alarm reporting to the central office, modification of the system may create problems with the agreement. You may be able to find an equivalent commercial sensor on E-Bay for a good price if you insist on replacing it. Personally I would stick with a commercial PIR for this application.
You only need to connect four wires for a burglar alarm motion detector to work properly. “GND” and “12V” are voltage for the motion detector - 12V is the positive, GND is the negative. “ALARM COM” and “ALARM NC” connect to the zone input on your burglar alarm panel. Typically, there is no need for polarity because this is a simple switch - with no motion in the area the switch is closed, motion is detected the switch opens.
“TAMP1” and “TAMP2” are connected to a mircoswitch within the motion detector housing that activates your alarm if someone were to remove the cover - called a “tamper.” If anything is connected to these terminals, you can simply twist the wires together, complete the circuit, and cap them off. It is not necessary to utilize the tamper feature and is often not recommended.
But, I would avoid any type of “inexpensive” replacements for your alarm system motion detector. One of the major sources of false alarms in burglar alarm systems tends to be the motion detector. Installing a motion detector not specifically designed for burglar alarm use may end up causing false alarms with your system and depending on your jurisdiction, fines from the local police department from the dispatching of those false alarms. Most burglar alarm motion detectors are designed to limit false triggers - so I would suggest going that route.
I did a quick search and could find a reliable motion detector listed for burglar alarm use for $25. Easy enough to replace yourself, just make sure you seal up any penetrations in the housing where the wire and screws go through. Spiders love to make homes in these and that will also cause false alarms.
Alternately, $89 may not be a bad price for your alarm company to replace the detector if that includes the labor. Nobody wants to work for free after all! Plus you can get them to quickly check your system to make sure it is communicating properly to your monitoring company and everything is working correctly.
I’ve read that bipolar transistors are current devices and MOSFET transistors — like old-fashioned vacuum tubes — are voltage-operated devices.
Although I understand the distinction conceptually, what does that mean from a practical perspective? For example, does this mean that bipolar are best for high power applications and MOSFETs are best for low voltage applications?
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Mosfets come in many flavors: there are N-type and P-type of course but there are enhancement mode and depletion mode. Enhancement mode are off at zero gate voltage; you have to apply a positive gate voltage for N-type or a negative gate voltage for P-type to turn it on. N-type mosfets are available with drain voltage ratings from 30 V to 800 V or more. Most mosfets are designed for switching; an 800 V, 10 amp device would quickly burn up unless it could turn on and saturate even quicker.
Mosfets are characterized by their saturation resistance which can be very low (like .01 ohms). Bipolar transistors on the other hand, are characterized by their saturation voltage which can’t get as low power as mosfets. The threshold voltage (the point where the transistor just turns on) is not well controlled so you can’t really know what the drain current will be at a particular voltage. That makes it difficult to design a linear circuit. I avoid that problem by using bipolar transistors in linear circuits, or using pulse width modulation in a switching circuit which can be filtered to produce an analog signal.
Depletion mode mosfets are on at zero gate voltage and you have to apply a negative gate voltage to N-type to turn it off. Junction fets are also depletion mode devices and the zero gate voltage drain current is not well controlled so they are usually binned and labeled so you can have some idea of what you are designing with.