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  1. dhwilliams

    How can I multiplex a digital signal on a DC line?

    Some baby alarms/intercoms use the mains to connect the 2 ends of an audio link. I think these modulate a high frequency with the audio signal and the high frequency is injected onto (and taken off) the mains wiring via capacitors. I am sure I have seen a similar principle used over low voltage...
  2. dhwilliams

    can a resistor solve this problem?

    buzzp, I agree that in a "normal" zener circuit, failing open-circuit would allow the full voltage to be applied to the load but when the zener is in series with the load, as I described earlier, it would actually disconnect the voltage. It is the short-circuit failure that would cause the full...
  3. dhwilliams

    can a resistor solve this problem?

    Like most modern semiconductors, Zeners are very reliable especially when they run cool! I don't know what the statistics are but I would assume that it could fail as a short-circuit so you need a way to protect your sensor. Protection circuits can get more complicated than the circuit you are...
  4. dhwilliams

    can a resistor solve this problem?

    A 5W Zener of this type in free air should easily handle the 2W without getting so hot it shortens its life - if you want to run cooler, use a stud-mounted zener on a couple of square inches of aluminium but bear in mind that the aluminium is at the same voltage as the stud terminal. You should...
  5. dhwilliams

    can a resistor solve this problem?

    If you fit a 440 ohm resistor in series with your sensor (2 x 220 ohm 2W in series) the sensor voltage would be about 20V with the relay on and 28V with the relay off. These both fall within the 18-30V range so, as long as the sensor can tolerate the change in voltage when the relay operates, it...
  6. dhwilliams

    Accelerometer Sizing

    The displacement is difficult to measure because the payload platform is moving in a semi-circle but I guess it could be up to .05 inch so, by your examples, a 1.2 G device might be a reasonable first attempt. I've requested a sample ADXL213AE accelerometer from Analog Devices Inc which should...
  7. dhwilliams

    Accelerometer Sizing

    I have designed a machine for my employer which has a radial arm 70mm long which oscillates through 180 degrees taking about 5 seconds to go from left to right and back. The "payload" on the end of the arm can be seen to vibrate (probably at about 15Hz) and too much vibration can cause...
  8. dhwilliams

    PCB board protyping

    I've used ExpressPCB with great success. Two 3" x 3" boards will cost you $92 if you are in the USA or $109 if you are in the UK (like me!) You download their PCB Design Software - very easy to use -and draw your layout. It includes online pricing, which is where I got the above prices. Once...
  9. dhwilliams

    Help needed on multiple relays

    ...will take about 36mA (I = V/R)and if you remove the LED but leave the 1K resistor the chip will supply just under 12mA so you need a gain of 36/12 * 5 (the 5 is because the quoted gain of the transistor, its hFE, is the small signal gain not its dc gain). So you need hFE=15. I would use about...
  10. dhwilliams

    Stopping a Shaded-Pole Motor

    Thanks Jeff and OperaHouse, it looks like it's now worth programming a PIC to do the timing - I hope 12V is high enough to stop the rotation in the right place because we have 12V power supplies as stock items and after being energised for an hour the coil was only slightly warm. I will probably...
  11. dhwilliams

    Help needed on multiple relays

    If you can do a bit of electronics, here's a link to a suitable circuit - http://www.zen22142.zen.co.uk/Circuits/Misc/quiz.htm
  12. dhwilliams

    Stopping a Shaded-Pole Motor

    I need to update the electronics in some machines we have sold and one of the things the existing control circuit does is to stop a 240Vac 50Hz 40W Shaded-Pole motor/gearbox at a specific position when signalled to do so (slotted optoswitch detects slot in disc on output shaft). If this was a...
  13. dhwilliams

    transfer switch problem

    I agree with IRstuff, a manual switch will not work and I think there is more to it that just adding pull-up resistors. Google for "KVM switches" (Keyboard, Video, Mouse) - these have built-in electronics to fool the computers into believing they still have the 3 devices connected. I used one...
  14. dhwilliams

    Help designing switching circuit...

    May I add a cautionary note, I once designed a logic circuit to replace a broken sidelight/headlight switch on my car with (debounced) push buttons which operated high current relays. It worked fine on the bench but when I wired it into the car I found that starting the car triggered the logic...
  15. dhwilliams

    Need to Sense DC current draw.

    Some years ago I fitted an anti-theft device that worked as follows: When I turned the ignition on I then operated a hidden momentary action switch. This operated a double-pole relay which latched using one set of contacts. The other contacts were used to enable the 12V ignition coil circuit...
  16. dhwilliams

    Need to Sense DC current draw.

    logbook, You must have been writing your reply while I was writing mine - I should have remembered the reed relay trick! (I'm old enough to have used them...) However, skiier said that the current was 8A so he may need more turns and he should only use one of the lead wires - unless he coils...
  17. dhwilliams

    Need to Sense DC current draw.

    Disconnect the negative supply wire from the lighter, reconnect via a 0.1 ohm 10W resistor. Connect an NPN transistor's emitter to the negative supply wire and connect the base via a 1K resistor to the other end of the 0.1 ohm resistor. Connect a 12V relay coil from the positive to the collector...
  18. dhwilliams

    I cannot view PDF Files on my computer

    Un-install Acrobat Reader, download the latest version (for your operating system) and install it. That should sort it! Dave
  19. dhwilliams

    Need UK Supplier For Expansion Card Screw

    Thanks zeitghost, I hadn't thought of CPC (obviously!) Unfortunately it's now too late to use these in the equipment we are selling, I had to use Stainless Steel Self-Tapping Screws from Rapid Electronics but I'll definitely bear them in mind for my next project. Best Regards, Dave
  20. dhwilliams

    Why is 110V safer than 220V?

    I'm no expert in this field but I believe that power tools used on site use a 110V isolation transformer with an earthed centre-tap. This means that the highest "shock" voltage (with respect to ground) is only 55Vac. This is obviously safer than 110 or 220Vac. Dave

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