how to build a 555 timer circuit for 5 seconds - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: July 25, 2018, 09:57:21 am
content: Hello everyone..I'm working on making a simple 555 timer circuit to power another circuit for one of my project. The timer should be on only for the first 5 seconds and then should be off for the rest of the operation. during the on time it should drive a mechanical relay and after 5 seconds the relay should be turned off as well. So could anyone help me out with a suitable circuit diagram for my application??

author: sokoloff
author_rank: Super Contributor
timestamp: July 25, 2018, 10:22:53 am
content: Try something like this:http://tinyurl.com/ycfydrey(If that short link breaks, here's thelong link.)

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2018, 01:59:15 pm
content: Quote from: sokoloff on July 25, 2018, 10:22:53 amI know it's just a simulation, but the 555 will not be able to supply the current of that lamp.Here's how it's done in LTSpice. A diode is added across the resistor, to discharge the capacitor, when the power is disconnected.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 25, 2018, 03:07:56 pm
content: Quote from: Hero999 on July 25, 2018, 01:59:15 pmThank you very much Sir for your reply. I have some doubts regarding the circuit and could you help me out??1)This timer is used to disconnect another circuit board for the first 5 seconds. the power supply for the timer is taken parallel from the other circuit board and there is no separate switch for the timer. what i want is that after the 5 seconds the timer circuit should be turned off through out the use of the other circuit until the switch for the other circuit is again restarted. Does this circuit provides this function??2) on reading the datasheet its found that the max working voltage for the 555 is 16v and however my supply voltage is around 16.8(4s Li ion cell). So a linear regulator 7812 would be needed at the input right??3) on the right hand side W1 and load 10R represents the switching side of the my relay right ??

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2018, 03:41:06 pm
content: Quote from: Adhith on July 25, 2018, 03:07:56 pmYour description is unclear. Do you want delayed on, or delayed off? As drawn the circuit will turn the relay on for the first 5 seconds, after power is applied, then turn it off. If you want the relay to be off, when power is first applied, then turn off after 5 seconds, then the relay coil and D2 should be connected between the 555's output and +V.Yes, that will work, but you might want to use the LM7808, to give more headroom, although the 555 will still work even if the LMM7812 stops regulating, so it won't be a problem. Another solution is to use the ICM7555, which will work up to 18V but it can't source as much current, as the classic NE555. It depends on the relay.Yes, the relay is modelled, for the purposes of the simulation, as a current controlled switch, with the input being the current through a 10H inductor, with a series resistance of 500R.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 26, 2018, 08:25:32 am
content: Quote from: Hero999 on July 25, 2018, 01:59:15 pm1)my apologies for making things confusing and unclear. What I want is the relay to be turned on for the first 5 seconds and then turned off until the switch is again restarted. This circuit could do that right??2) I'm thinking of moving forward with 7812 and NE555. That would work right??3) So i just remove the W1 and Load 10R and replace it with my actual load on the relay right??Also one thing since i'm a bit curious. what is actually happening to the 555 timer in this specific circuit after the 5 seconds?? Is the 555 acting as a completely open switch after the 5 sec or it is always drawing a small amount of current but bypassing it to the ground without powering the relay coil after the 5 sec??

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2018, 08:49:11 am
content: Quote from: Adhith on July 26, 2018, 08:25:32 am1) Yes.2) Yes.3) Yes.The 555 is acting as a Schmitt trigger. When the 555 is first turned on, the trigger and threshold pins will be at 0V and its output will be near +V, causing the relay to turn on. C1 will charge via R1. When the voltage on C1 exceeds2/3of the supply voltage, the 555's output goes down to 0V and the relay turns off. D1 allows C1 to discharge, through the rest of the circuit, when the power is removed.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 26, 2018, 12:54:27 pm
content: Quote from: Hero999 on July 25, 2018, 01:59:15 pmOk got it. so it will act as an open switch right ??after the 5 seconds there is further current flow to the 555 timer right??

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2018, 01:10:14 pm
content: Quote from: Adhith on July 26, 2018, 12:54:27 pmYes, then power is applied, the relay will turn on for the first 5 seconds, then turn off, becoming open circuit.