Amp shutdown control

[washed]

Some NPN BJT and a few resistors should do it, i think?

[tmh-alex]

Works. Guess you could crank up R1, R2, and R5 by a magnitude of ten. Should still be robust enough against noise and reduced power consumption. I wonder if R2 is needed at all (defeats the floating property, doesn't it)?

[washed]

Good points, thanks! That looks good as well:

• removed R2
• R1 to 10k
• R5 to 100k

Low level seems to be around 80mV; should be totally fine for a TTL input.

[tmh-alex]

Waaait, aren't we overlooking the obvious? Is R5 even needed, either? According to the datasheet the base cutoff current is 50 nA, putting the upper limit of R1 to 52 MOhm. With 1 Mohm we'll surely be on the safe side with a base current of 2.6 mA. We can afford that :)

[washed]

I have to admit i am super rusty (ha!) on discrete stuff. It seems fine without R5 and an R1 up to ~20k from some quick sim.

[tmh-alex]

No worries, I learned and used this stuff at some point and I had to look at the diagram three times.

[washed]

Used a BC547C (first NPN i found in a bag^) and a 100k between base and gpio. works a treat. One potential (pun intended) issue: we need to tie two previously separated grounds together. We'll need to see if this has any adverse effects; the schematic assumed both grounds are already the same net.

[tmh-alex]

If that's a problem, try a 0.33 uF capacitor between the transistor base and the raspi ground. This should make a first order filter with the cutoff frequency at around 25 Hz (assuming R1=20 kOhm)

[washed]

Much complex

[washed]

Software side is implemented by this: https://github.com/washed/drempelbox/commit/2e209eeeb605e9b5bdb4f3ad1ef7510ee3e0d448 we'll need to decide on when to switch on/off at some point. But now it is possible to do so :)

[washed]

See #11 for an updated version of this

[washed]

Happy with this after initial testing. Also see #11, #5.