How to get 10 seconds timer (issue with caps getting late discharge)

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venkir

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I’m a beginner. I wanted to have a basic timer like circuit to have LED on only for 10 seconds after I switch on the circuit (only inexpensive components without MCUs).

I found some delay on circuit and added 1 more TR to have inversion (see the attached pic).



I played with the R1, R2 & Caps to get 10 seconds ON after a long struggle.

But, when I switch OFF and ON the circuit next time, the LED is ON only for 3 seconds (caps already has some power). It depends how long I wait after OFF, for caps to discharge via R2.

But, my requirement is to get ON for 10 seconds all the time without even waiting. I tried a lot and failed to achieve what is needed. How can I get same 10 seconds LED ON and OFF after that till I switch off & ON again.

I realized that R2 must bigger than R1 or else caps wont charge enough to trigger T1.

Please help me to have it with few components. I wonder if I can get it with 555.
 
You can extent the time by adding a zener diode in series with the base of TR1 (anode towards base). Try about a 3.3V zener to start. You could also add two or three regular diodes in series with the base (cathode towards base).

To completely and rapidly discharge the capacitor you could use a one pole of a DPDT switch for that. You could also add an NPN transistor to discharge the cap. For this you could use a SPDT switch to turn on the transistor when the cap charging circuit is turned off.
 
your switch is in the wrong place... use it to power up the whole circuit or turn it off, and then you can clamp your cap to the +v rail so that when you turn power off, as vcc goes below cap voltage, the LED will automatically discharge the cap for you...

otherwise, you don't need a DPDT switch, a SPDT will do, and leave the common tied to the 10k, and switch the cap from vcc to ground to rapidly discharge the cap when turned off.
 
SPDT Switch option:



change R2 to 100 ohms to discharge the cap and tie the base of the transistor to ground when the switch is in the off position.
 
Delay off

Or you could try somethig like this:

I think you will need a bigger capacitor to get 10 seconds.
 


With the way the circuit is drawn, when you turn the switch off, as soon as the cap discharges, the LED will relight... I suspect the switch is in line with the 5v??? If so, adding a single diode to clamp T1 base (top of cap) to vcc will reset the cap when you turn power off...
 
ronv said:
Or you could try somethig like this:
View attachment 71436
I think you will need a bigger capacitor to get 10 seconds.
Click to expand...

This circuit will do just the opposite of what he's looking for...
1) When switch closes, LED will go off almost immediately as the cap will charge up very rapidly, with no inline resistance, it will only be slowed by circuit impedance and limit of power supply to supply current (and capacitor to receive it, of course).
2) When switch is opened, LED will stay off until cap discharged and then come on without switch closure.
3) With larger cap and same discharge resistor, cap will take longer to reset, so switch will have to stay off longer to get time correct.
 
I wonder if I can get it with 555.
Click to expand...
Yes, you can. That would be my choice. There are gazillions of schematics online for doing this. The 555 datasheet will also give you enough info to do it.
 
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