## Beginner wanted a delay timer trigger with 555 timer.

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Peacock
Posts:5
Joined:Wed Feb 22, 2017 12:57 pm
Beginner wanted a delay timer trigger with 555 timer.
Hello everyone!

I have the following scenario.
When a 9v battery is switch on, it waits 5 seconds, then a reed relay is activated.

I am a complete beginner in this so please explain to me as if I'm an idiot.

I only know about capacitor, relays, and diodes at this point.
Am I making any sense?

thank you,

Junaid_Shahid
Posts:110
Joined:Tue Feb 14, 2017 9:00 am

### Re: Beginner wanted a delay timer trigger with 555 timer.

Peacock wrote:
Wed Feb 22, 2017 1:42 pm
Hello everyone!

I have the following scenario.
When a 9v battery is switch on, it waits 5 seconds, then a reed relay is activated.

I am a complete beginner in this so please explain to me as if I'm an idiot.

I only know about capacitor, relays, and diodes at this point.
Am I making any sense?

thank you,
You can use a 555 timer in a monostable mode to get the desired effect. Check out this page, under monostable, and using the circuit modification mentioned in "Power-on reset or trigger" (cap in parallel with power switch)
555 Timer circuits

The basics of why this works is detailed in he Power-on reset section, but the principle is an R-C charging circuit. A series resistor and capacitor can create a voltage-based delay. The resistor slows down how quickly the capacitor can charge up. As the charge in the cap slowly builds, it's voltage rises. When the voltage hits a certain level, you can sense it and use it as a trigger (that's what the 555 timer is doing... sense voltage level and triggers).

An R-C charge circuit will reach 63% of the final voltage within 1 time-constant, which is defined as tau=R*C. So if you have a 5V supply, R=2Meg and C=2.5uF, then the capacitor will reach 63%*5V = 3.15V in tau=2000000*0.0000025 = 5 seconds. I'm assuming the trip-point of the 555 is 3.15V, which should be pretty close. You can pick any value of R & C, and experiment with the length of the delay.

Recommended Tutorial: Relay Control Using 555 Timer in Proteus ISIS