[SOLVED] [Help] Simple delay circuit

Hey everyone

What I'm trying to do: As soon as we have power, run the relay for a few seconds (adjustable by changing the Capacitor if I'm not wrong)

Note I'm not very familiar with electronics so I'm probably making a huge mistake here!


The version that works:

As soon as I click the button the LED stays on for 2 or 3 seconds.

Q: When I'm NOT pushing the button why does the multimeter shows 2V between the V+ and the Transistor Emitter?


The version that doesn't work:


Here multimeter shows 0V between the V+ and the Transistor Emitter and when i push the button something like 0.07V


Thanks In advance, I will greatly appreciate every explanation in order to improve and learn from my mistakes

Mywk said:

Hey everyone

What I'm trying to do: As soon as we have power, run the relay for a few seconds (adjustable by changing the Capacitor if I'm not wrong)

Note I'm not very familiar with electronics so I'm probably making a huge mistake here!


The version that works:

As soon as I click the button the LED stays on for 2 or 3 seconds.

Q: When I'm NOT pushing the button why does the multimeter shows 2V between the V+ and the Transistor Emitter?


The version that doesn't work:

Here multimeter shows 0V between the V+ and the Transistor Emitter and when i push the button something like 0.07V


Thanks In advance, I will greatly appreciate every explanation in order to improve and learn from my mistakes

Click to expand...

Your circuit is wrong in two points: 1. The transistor is reversed- the NPN transistor allows only collector to be connected to +5V.
2. To open the transistor for several seconds you must allow the base current to flow. It cannot flow through the capacitor as shown.
By now I think you have killed the transistor. Use a new one.

If you connect your NPN transistor correctly, emitter to ground and collector to +5V through the relay coil, then use a 5 K resistor from +5V to the base, and remake your circuit as follows: From +5V, connect first a trigger switch, then 1 kOhm resistor, then a 10 uF capacitor to ground. Use this capacitor as a source for the base current- connect the base resistor to the positive capacitor pin.Once you trigger the switch, it will charge the capacitor and open the transistor, thus the relay will hold for several seconds. Once the capacitor discharges, the transistor will shut down as well as the relay.
The hold time can be set by capacitor value and the 5K resistor. Do not use smaller than 5kOhm value. Use a relay with not more than 100 mA coil current at 5V, otherwise you will kill the transistor again.

Actually I had it half way done, I messed up when making the first schematic.
If I only click the switch then release it, the transistor stays on after about 2 seconds, but this is not what I want.

The switch will be always ON (removed from schematic), what I want is: As soon as I have power I need that the transistor stays OFF for a few seconds, so it only triggers when the capacitor is fully charged.

-- Edit

I think I got it, now it takes about 5 seconds to start the transistor, thanks a lot for your help