Hi! i have a transformer powering a art piece of mine, and it is to go on display on a school show, so i have decided to make up a simple 555 one shot system so when a button is pushed, a relay comes on and stays on for 10 seconds then clicks off until the next button push. i found the monostable circuit i needed here: HERE and breadboarded it and it worked fine, the relay is justed tagged off pin 3 with a back EMF diode, like bill bowdens design: HERE as he stated, it can source 200ma... anyway, to cut a long story short, it is very sensitive (just touching the trigger pin sets it off and when the relay is attached (this is all soldered and built in now) it latches on once pressed and only by disconnecting the relay and reconnecting does it reset. i put a 22000uf supression cap on the power lines to see if it had fluctuations but no effect, the power is 7v made from a center tap type rectifier / 4.5v transformer with a 2200uf cap. steady 7v. (tested with scope) so why is this thing so over sensitive? if i tag a LED off the output pin (instead of relay) it works fine (but is still over sensitive - just goes off after 10 secs)! i think the relay is somehow triggering it (as it is so sensitive).. but how? for simplicity, i built the 555 part on a RS 434-071 board (555 board) and wired it exactly as that schematic! why is this so sensitive.. thanks in advance, Alex
Ive never really got as far as 555's.. but from other posts on here i would guess either try another value for pull-up resistor, or maybe it needs de-bouncing? - unlikely becuase of the 10sec delay?
This may help reduce trigger sensitivity. You are not alone. I also wonder if the momentary-off push-button method shown further on would be less prone to the problem.
What current do your relay and LED take? Let's say a load puts 100mA on your 2200uF cap during the discharge cycle then the voltage droop is: dV/dt = I/C = 100mA/2200uF = 45V/s Of course your cap is recharged to its peak at least every 10ms (assuming 50Hz full wave) so at worst the droop is 0.45V. It will be more like 0.1V, but it could already get in the way of your switching thresholds. You should see the ripple on the scope with AC coupling on the input. The droop could explain the fact that the circuit works with the LED but not with the relay. The LED runs on less current, so there is less droop.
