Cutting power to a circuit by time elapsed.

Question

I had an idea the other day and I wanted to ask about the specifics of it here.

So, I have a few guitars that use active pickups, meaning they require batteries (usually 9v) and one annoying aspect of them is if you leave a cable plugged into the guitar’s output jack, it’ll continue to drain the battery. Normally this isn’t a huge issue, but there are times when I forget to unplug it or I leave it plugged in for a few hours when I go to do something else.

Anyway, I thought it would be cool if I could add a circuit that could cut the power based on how much time has elapsed without a signal. It would need to be fairly small, as it needs to fit in the guitar’s control cavity and preferably be independent of any of the guitar’s circuitry. I don’t want the signal chain going through anything that could affect the tone. You’d think manufacturers would include something like this already, and maybe they do, but the 3 guitars I have with active pickups don’t have it.

Since I’m a complete novice when it comes to designing circuits, I thought I’d ask here. The only thing I could think of was maybe a 555 timer, but I know next to nothing about them. I remember Ben did something similar on an old episode with a baby monitor and music player, but he used a lot of the existing circuitry from the monitor if I recall correctly.

Well, that’s my idea. Thanks for any help or advice.

dougw · Answer

If you aren't into buying a computer and programming it, you could try a circuit like this: After the circuit is turned on with the momentary ON switch it will stay on for about 5 minutes after the pickup has gone quiet.

cookekeyes · Answer

I‘m starting to see why manufacturers don’t include this in their active pickup circuitry by default. It adds a lot of components and complexity to fix something that is already easily fixable by just removing your guitar cable. It does seem quite ambitious for a novice like me, but I like working with this stuff anyway. It’s like electronic legos. Since I already own all the resistors and protoboard, and the ICs are pretty cheap, I might go ahead and order some and give this a shot. Thank you all for the advice.

genebren · Answer

Doug, I don't know if I am confused about the timing, but I come up with something like turn off in 3 seconds after last signal. I am getting a frequency of oscillation of ~ 5583Hz and a count of 16384, yielding a delay of ~3 sec. Gene

dougw · Answer

Thanks genebren … I'm glad someone is "doin the math" - I didn't spend much time on calculations - until someone wants to build it....the circuit is easy to adjust to a larger delay period with larger RC values.

michaelkellett · Answer

Almost certainly best to base it on a tiny processor. There will have be some connection to the guitar to check the signal. I recently used some of these for an emergency patch - STM32G031J6M6, ARM processor with ADC in 8 pin SOIC package - can run at 2MHz clock and use about 0.2mA - which I guess will be a lot less than the normal operating current. Once the power has gone off the processor can sleep and draw less than 1uA (battery run down in 500,000 hours) - other bits may use more power ! You could use a TI TPS22810 for the power switch. You'll also need a micropower regulator for the processor. I've attached a picture of my STM32G031J6M6 application to give you an idea of achievable size: I'm not offering to design this for you - I have time to help a bit but not to do it all, it would be an ambitious project for a novice - you might be able to team up with some other people. MK

cookekeyes · Answer

Thanks a lot for the schematic! I think 5 minutes would be the absolute minimum I’d be comfortable with. Ideally, it would be around 10 minutes after no signal. So to change this time, I would need to change the resistors correct? But which ones? Also, is there a way to do this without the momentary switch? I’d prefer it to work by itself without me having to press anything to activate the circuit. If at all possible, I’d like it to function solely from the guitar signal (or lack thereof) I can read the schematic, but like I said, I’m a complete novice when it comes to electrical engineering. **Edit** After rewatching the “Smart Switching Headphones” episode, I noticed Ben basically built exactly what I’m looking for, except I want my circuit to cut power instead of switching to another input. It was driven by sound, similar to how I want mine to be. He used an AGD436 and a 555 timer, but also utilized a lot of the existing circuitry in the baby monitor. The main thing I don’t know how to do is how to trigger the circuit to activate using the guitar’s sound output.

dougw · Answer

R1 and C2 control the oscillator. The period is about 0.22 x R x C and the counter is counting 16384 cycles to give the delay. R1 could be 1M and C2 could be up to about 0.22 uf. Which would provide up to about an hour if my calculation is right. You could use different outputs from the counter if you want to cut the time by powers of 2 without changing any components. So one connection could be set to 5 min, another one could be set to 10 min, and a third could be 20 min. Or you could use a potentiometer for R1 and make the time adjustable. Incidentally, the output will turn off half way through the period of the counter pin, so the example above might be half an hour instead of 1 hour.

michaelkellett · Answer

You have a bit of a problem triggering the power from the guitar pickup signal. The built in amplifier will be off and it may load the pickup under such conditions - you need to check this. Guitar pickups can provide quite large signals but ones used with amplifiers may not - you need to check the signal you get at the input of the amplifier when its off and you strike a string at the level you want to activate the system. Then you need an always-on bit of circuit to start up the power timer - this should consume an average current of less than 50uA so that it doesn't shorten the battery life too much. Doug's circuit shows an input resistor of 100k, that may be too low low for guitar purists - depending on pickups an input impedance of > 2M may be needed. Suitable micro power op amps exist - Microchips MCP6421 would do. Have you thought about one of those push switches in the hallways of cheap blocks of flats - where you press it in and after a while (the time it takes to get halfway up the stairs) it pops out and the power goes off MK

Cutting power to a circuit by time elapsed.

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