- 555 delayed start timer 3.GIF (5.7 KiB) Viewed 19543 times
Here is a circuit I made some time ago that will do what you want. It consists of a 555 timer for the delay followed by a second 555 for the ‘on’ period.
When you first switch on C2 is discharged so it triggers on IC1and ensures that IC2 starts up in reset mode. C1 will then start to charge via R1 until the voltage on pin 6 of IC1 reaches 2/3 of the supply voltage, then the 555 will reset. The falling voltage on IC1 pin3 now triggers on IC2 via C3; the relay switches on, and C4 starts to charge via R4. When pin6 reaches 2/3 of supply voltage the relay switches off. That ends the cycle until power is switched off then on again. In your case you can replace the relay with a number of LEDs in parallel in place of the relay.
The time period of the 555 delay is a function of R1 and C1.
R1 (in megohms) x C1 (in microfarads) is known as the ‘Time Constant’ (TC). In the circuit, TC = 0.470 x 10 = 4.7seconds. The time taken for C1 to reach 2/3 of the supply voltage is 0.72 x TC seconds. So the time will be 4.7 x 0.72 = 3.38seconds.
If you use 22µF and 330K the time will be 4.7secs, and if you use 25µF and 270K the time will be 4.8secs.
You can easily work out values for 5secs and 10secs by varying C1 and R1. There are two provisos:
1. Electrolytic capacitors have variable leakage currents, so if you want accurate repeatable times then use bead tantalum capacitors.
2. The value of R1 should never exceed 2megohms.
You can use the same calculations to get the relay-on time from C4 and R5.
R3 is there to discharge the capacitors when the supply is switched off.
I hope you find that useful and come back with any questions