I wired up a 556 (two 555 timers on one chip) as follows:
Timer A: astable circuit with equal mark & space width, R = 267 ?, C = 1000 µF (electrolytic with 25 V rating), output: 320 ? in series with an LED, also tied to the reset pin on Timer B
Timer B: same astable timing arrangement, R = 100 k ?, C = 10 nF (non-electrolytic), output: piezo speaker in series with a resistor
I powered this concoction with a 9 V battery, and got more or less the expected result: the speaker alternates between silence & a high-pitched noise, and the LED lights up when the speaker is on. The first few cycles of timer A are a bit irregular (it stays on for a few seconds before first switching off), but then it evens out.
When I supply the whole circuit with 3 V (two AAAs) instead, however, (and reduce the resistance in series with the LED accordingly), the LED stays on all & the speaker whistles all the time.
Is tying the output of timer A to the reset pin of timer B the most correct way to switch B on and off with A?
One of the books of 555 circuits says that electrolytic capacitors only work properly as capacitors above 10% of their rated voltage. Is this why timer A doesn't work on a 3 V supply?
Even with the 9 V supply, the LED doesn't go completely off when the speaker does --- it's just noticeably dimmer than when the speaker is on. Why?