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I'd like to build a keyboard emulator, and I've found three links that detail the process: 1, 2, 3. Here's the rough circuit:

The thing is, across the various articles I've read, people are using different voltage power supplies. All use DC. Most use 12v, but some reference 5v supplies. Others use 9v or 10v.

My question is: how do I select a voltage to use? I'd like to create the above circuit approximately 5 times, on the same supply.

Batteries would probably be easier - would it be possible to use a single 1.5v AA battery? If not, how many would I need? When would I use AAA's? How long could I expect the batteries to last?

I can of course change the capacitors and relays out, to match the voltage of the power supply.

Some general guidance would also be appreciated! Some background is in my other questions.

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It may be possible to use a 1.5v AA battery if you are able to find relays that operate at that voltage. Relays will be the most costly part of this setup, so I'd start searching for relays that you think would operate well for your purposes. The specifications and price on the relays should be the most important consideration in building this. You'll want to find them rated at very low voltage levels as well as having comparable closed switch resistance values to what is discussed in the articles.

Looking at some relays on digikey here: http://www.digikey.com/product-search/en/relays/signal-relays-up-to-2-amps/1049448?k=relay there seem to be many relays that have a Turn-On voltage between 1.05 and 1.5 V. If you get one of them, then you can choose your circuit to operate at that low of a level. In short, find a relay you like/have/can get cheap and then build the rest of your circuit around them.

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Thanks to @horta's answer, the cheapest relays I've found are 12v. I'm going to use these that I have lying around (which has both a 5v and 12v DC output):

Male Molex UK HDD power supply UK kettle cable

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