This time I built two astable circuits, using different designs (from the same book), on the two timers in one TI NE556 chip.
Timer A:
reset pin --- Vcc discharge pin --- not connected
control pin --- not connected
output --/\/\-- trigger & threshold --|(-- ground
Timer B:
reset pin --- Vcc
Vcc --/\/\-- discharge --/\/\--- trigger & threshold --|(- ground | | |--|>|--| 1N4148
To avoid confusion (as generated by my previous post), I've used pin names rather than numbers. The 556's ground & Vcc pins are connected normally to a 9 V battery. The output of each timer is a 470 ? resistor in series with an LED (so 20 mA on each output, well under the 200 mA recommended limit in the specs).
Circuit A provides equal mark & space times, with a total of 1.4×R×C. Circuit B is supposed to have independent mark & space times, each
0.7×R×C.I've wired them up with R=6.9k ~ (for A and for each resistance in B) and C=100 µF (on each timer), which gives a theoretical mark+space of
0.96 sec for both circuits.At some point, I noticed that application note PDF for the Philips version of this chip strongly recommends connecting the control pin through a 10 nF capacitor to ground if you're not using the control function (to protect the timer from noise). So I wired one between each control pin & ground.
I've only seen circuit A in this book; other sources only give circuit B and say to use the same resistance in both positions if you want equal mark & space. Is circuit B better than A? Why?
The two circuits seem to interfere with each other, and the control bypass capacitors affect the time as well. Lacking fancy equipment, I measured the mark+space time by timing 10 pulses with a stopwatch & dividing, and got these results:
with the control bypass capacitors:
timer A with B off: 1.1 sec timer B with A off: 0.9 sec timer A with B on: 0.6 sec timer B with A on: 0.6 sec
without the capacitors:
timer A with B off: 1.2 sec timer B with A off: 1.0 sec timer A with B on: 1.3 sec timer B with A on: 0.9 sec
Should they interfere this way, or have I goofed something up?