Alternative long time 555 monostable

[bertus]

Hello,

I got an article in Funk Amateur , a german magazine about a long time 555 monostable.
It uses the discharge transistor to charge the capacitor and a parallel resistor on the capacitor to perform the timing.
In this way you can even make long times upto 0.5 hours:



Sorry for the german text, but the schematic will be clear.

Bertus

 

[AnalogKid]

Actually, the circuit on the right will have a *shorter* timing period than the circuit on the left, by approx 0.1% (1 K / 1 M, assuming that Vcesat is the same in both circuits). But as a benefit, the added 1 K resistor will protect the Discharge transistor from a current spike that could be destructive over time.

ak

 

[Alec_t]

If the right-hand circuit gave a substantially longer timing period than the more conventional left-hand one, using the same timing components, it would be very handy. However, it doesn't. The problem in getting long timing periods with either circuit is leakage current in high value timing capacitors.