555 timer long delay - input swap circuit change

[u/Staplesofficewar]

Hi All - been a while since I tried to work on circuits at this level, but the attached is close to what I need. This allows long delays (not necessarily accurate but doesn't matter in this application.) Pulled this off of: https://homediyelectronics.com/basic/longtimedelays/

Now, this circuit, the output starts off high, when the button is pressed, it goes low for the time designated by RC, then goes high again. If the button is on all the time, the timer doesn't start.

I am trying to modify this and keep the long time delay, but instead of timing when power is removed (button off) I need to time when the power is ON. Like a typical monostable setup on a 555, but with the transistor controlled trigger that allows for long delays with low C.

I prefer to stay with 555 timers at this point for this application.

Any suggestion is appreciated!

[u/PointFirm6919]

I'm pretty sure all you need to do is remove the transistor like so:

[u/Staplesofficewar → u/PointFirm6919]

I might have to go this route if I can’t get the longer delays. Maybe even cascading outputs with multiple rc time constants. Ideally I would have a 2 hour delay before the output turns on after applying power. I understand that is hard to get with typical caps and resistors without something “fancy” like the transistor? I’ll probably start running up agains leakage current of the anyways, so another challenge

[u/PointFirm6919 → u/Staplesofficewar]

Sorry, I misinterpreted the question.

I believe this circuit could work:

In this case, pin 2 is normally grounded, so output is high. When the button is pressed, the voltage at pin 2 is set by the potential divider of R1 and R2, which should be greater than 1/3 of VCC, setting the output low.

When the voltage across the capacitor rises above the voltage across R2 and R4, the transistor is saturated and the potential at pin 2 is reduced due to the ground side of the potential divider now being R2 and R4 in parallel. This voltage should then be less than 1/3 of VCC, so the output goes back high again.

The voltage at the collector and emitter of the transistor should be roughly equal to make sure it acts as a switch rather than an amplifier, so it would be a good idea to make R1 and R3 equal and make R2 and R4 about 3/4 of that value, so the voltage at pin 2 goes from about 0.43*VCC to 0.27*VCC when the transistor is saturated, and you don't have to worry about any hysteresis from ambiguous values.

The part I'm not sure of is that the rate that the capacitor charges is determined by Rc, but I don't know what equation to use. It would also be good to use a DPDT switch for the button so that you can dump the capacitor's charge and connect pin 6 to VCC when the button is released.

Hope this helps.

[u/Staplesofficewar → u/PointFirm6919]

This got me real close. See link and pic. It works but sloppy… initial off - check. Charge cap then on - check. Power off - output off - check. Any improvements?

https://www.falstad.com/circuit/circuitjs.html?ctz=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-6q7G4nNYjMGE9O2V33x3rWcIA

[u/PointFirm6919 → u/Staplesofficewar]

In that circuit the capacitor takes a long time to discharge after the switch is opened, so when the switch is closed again the output goes high straight away. If you want the time delay to be the same every time, you can dump the charge to ground through the same switch:

Other than that, I'm not sure. Do you have any specific ideas for how you want it to behave differently to how it does now?

[u/motoware]

Not exactly sure what you mean...

Try this below. The second 555 is triggered by the first one, so the second LED is a delay ON circuit. You can adjust each 555 pulse low width

The Simulation is slow so I reduced the 100uf cap to 1uf...left side. Adjust values as needed.

You can adjust sim values by hovering over a part, then right click, edit, change value

https://tinyurl.com/2d5zkjpr

[u/Staplesofficewar → u/motoware]

Sorry I’ll try to be more clear. The circuit I showed is supposed to be a long delay circuit thus the transistor. The existing circuit, when powered on without the switch on, will immediately have an indefinitely long output. Once the switch is closed, the output will be on until the cap discharges. What I am trying to do it basically the opposite. I need the long delay circuitry but want the output to be initially off. When I provide power ( either to circuit as a whole or just the rc side) I want it to wait based on the RC value, then output until I remove the power to the rc circuit or the circuit as a whole. Basically needs to work like a normal monostable setup but with longer delays. Thanks