ne555 monostable timer problems

ne555 monostable circuit

I have a strange problem with the (classic and very simple) design of a timer with ne555.
The circuit is the standard monostable configuration, with DISCHARGE(7) and THRESHOLD(6) connected, RESET(4) tied to V+(12V), TRIGGER(2) connected to V+ through 10k resistor, 22uF capacitor between DISCHARGE and ground, 100n on CONTROL VOLTAGE(5) and ground, and 0->4M7 variable resistor between DISCHARGE(6-7) and V+.

The behavior is: as the trigger pulse applied,the output goes high and the Voltage on 6-7pins goes to 0 and then begins to rise because the capacitor charge through the var. resistor (ok so far). According to theory as the voltage reaches a threshold level (about 2/3 V+) the output must goes off, and the capacitors will discharge trough an internal transistor over the DISCHARGE pin. But no! In my circuit the voltage of the 6-7pin remains just below the threshold level (~8V) and stuck in this whit output high! (That it is true for var.resistor values>0M5). IF in this situation i give a trigger pulse, with the release of trigger from ground the output goes low! Again IF in this situation i touch the CONTROL VOLTAGE pin again the output comes back low!

It seems that there is the currents: [capacitor---->DISCHARGE(6-7)] == [V+ ---->var.resistor---->capacitor]. While the V on 6-7pins stays just below the threshold level. BUT WHY ? Any help appreciated.

ne555 monostable

The most likely culprit is the capacitor's leakage current, since it sounds like you are using an elecrolytic (22uF). The leakage can be about 1uA. All this current, plus the current that goes into the NE555 comparator input, must come from Vcc through the timing resistor. Since the timing resistor is large, the voltage drop across it can become significant and the voltage across the cap never reaches the high threshold. (Actually, it sounds like it's right about there, but there just isn't sufficient current left over to make the 555's comparator trigger. The transistors in the 555 comparator need some current to flow itto their bases, but what's left once you subtract the cap's leakage current is too low.)

To test the theory, reduce the resistor. If all works well, then the solution is to reduce the timing resisto and increase the timing cap. That is because the leakage current of a larger cap will not be much larger, but since the timing resistor is lower, things should work just fine.

monostable ne555

VVV already answered your question but here is an appnote,that you can use as a reference for future circuits.

ne555 monostable capacitor

this a reply adressing the problems of t_maggot...
i think u should use a pull up resistor in the trigger input...
about the pull up resistor u can consult the book of electronics by boghart.
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