555 timer circuit with latching switch instead of push button - Page 1
Forum section: electronics / beginners

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 22, 2024, 01:22:48 pm
content: Hi, I am trying to make a timer circuit using 555 IC but instead of push button, I need to use a toggle switch. When I press the switch, the signal goes high and triggers the timer, the output turns off after the time set by resistor and capacitor is over but the switch is still on. I found a circuit online but its doesn't work when I try to simulate it on proteus.This is the circuit I found online.This is not my simulation.

author: soldar
author_rank: Super Contributor
timestamp: May 22, 2024, 01:51:18 pm
content: Try something like this. Closing the switch gives a downward pulse to pin 2 but then nothing happen until you open the switch and close it again.You will need to modify the values to produce the timing you want.You can ignore or use the output inverter depending on your needs.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 22, 2024, 02:44:54 pm
content: Could you please explain what value does 100K capacitor mean?

author: themadhippy
author_rank: Super Contributor
timestamp: May 22, 2024, 02:56:08 pm
content: at a guess 100000pico farads,or 100nf or 0.1uf

author: soldar
author_rank: Super Contributor
timestamp: May 22, 2024, 04:00:27 pm
content: Quote from: themadhippy on May 22, 2024, 02:56:08 pmyes, sorry, I drew the left part myself so it would do what the OP wants and I just wrote the values in one format while the ones on the right are in a different format.My values are in pF.Note that the values I chose are off the top of my head so maybe can be tinkered with. I have not tested it but I think it should work well as is.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 22, 2024, 04:48:40 pm
content: Hey, Thanks you. I tested in proteus and this works fine. So, Here's the problem. I am trying to make a timer for my old Toyota's doors. So, that when I open the door, a timer starts for (t) seconds and lights up the LEDs that I have wired above the leg space. The door switch also connected to the overhead lights and probably gives out a +12v output which I am going to have to tap to trigger the timer. In the above circuit, the switch is shorting to the ground and this will not work. What would be the proper solution?

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 22, 2024, 05:34:14 pm
content: I tried to modify your given circuit in this way. This was also suggested on many other sites. But the simulations show only 2.4volts which is not going to turn a mosfet which I was planning to use, nor a relay. Is this only a simulation problem? What is the reason for voltage drop?I have attached the simulation.

author: themadhippy
author_rank: Super Contributor
timestamp: May 22, 2024, 05:55:34 pm
content: Quotebest double check that as it not unusual to have the switch on the ground,why run 2 wires when 1 and the car body will do

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 22, 2024, 07:31:04 pm
content: Here's how the door switch works.

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2024, 08:55:38 pm
content: Quote from: MrOmnos on May 22, 2024, 05:34:14 pmWhere's the 0V/earth/ground node? I suspect that's why the simulator is giving nonsense results. It would also explain why it wouldn't simulate the circuit attached to the first post, which should work.EDIT:Here's a simulation of the most recent design. It should work. I had to increase the value of the trigger capacitor. I suspect it's an artifact of the simulator, due to the time step being too long and it would work in real life with a lower value. It depends on how sharp the power on transient is.This is a case of user error. A simulator just performs calculations. Put rubbish in, get rubbish out.

author: soldar
author_rank: Super Contributor
timestamp: May 22, 2024, 11:18:50 pm
content: This should work with the standard door switch

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 23, 2024, 05:11:37 pm
content: Hey guys, I tried the above circuit provided by themadhippy and its works great. Now, I have tried adding a mosfet IRFZ44n on the output side but it doesn't seem to be working. The damn mosfet doesn't fing turn on.I tried simulating, it doesn't work on the simulator.I have attached below. I actually confidently soldered everything and its doesn't work.The timer is working properly but I think I am doing something with the mosfet. Before you say it, I have tried adding, gate resistor and pull down resistor (10K) to no avail.I regret going this path, should have just used an ATtiny at this point.Please, help me.

author: themadhippy
author_rank: Super Contributor
timestamp: May 23, 2024, 05:21:26 pm
content: Quotedid i? i deny everything,you cant prove a thing.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 23, 2024, 06:08:11 pm
content: I mean, by soldar. I am testing the circuit I built soldered and it doesn't seem to be working. The output on pin 3 is constantly 6..6V (rail voltage is 12) and timer isn't working. I am using a 500k pot with 1000uF cap. IS it the large capacitor that is not letting the circuit function properly?? What is happening. I have spent alot of time in this. I hate this.

author: shapirus
author_rank: Super Contributor
timestamp: May 23, 2024, 06:17:59 pm
content: so it works without the mosfet but doesn't work with it?if so, then first of all make sure you get your mosfet pin ordering right.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: May 23, 2024, 06:45:05 pm
content: Hi, I will try explain it properly. I simulated the circuit without the mosfet and it worked, so I went ahead and soldered it. IT didn't work so I thought mosfet was at fault, so, I disconnected all its pins and left the pin 3 of NE555 alone for testing. I started testing the timer I soldered and noticed that the timer wasn't working either. Pin 3 was always on and at nearly half the voltage of the rail voltage. When probing, I found Pin 3 and 5 were at same voltage and always on that (6.6v). So I disconnected the ceramic cap connected to the 5 and GND and since it was 0.1uF and I read somewhere that it might cause problem. After removing the cap I probed again and found both the pin 3 and 5 are now at slight below rail voltage that is 11.5v and always on.What could be the problem. How do I troubleshoot this?

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2024, 09:54:07 pm
content: You could go down the microcontroller route, but it's not much difference complexity wise. You still need a 5V regulator, a couple of decoupling capacitors, the MCU and a MOSFET.R1 needs a diode across it. When the power is removed, the power on pins 6 & 7 will exceed the power supply voltage, which isn't good for the 555 timer.Have you tested the MOSFET in isolation?Have you tried building the first circuit posted in this thread? It should work, whatever the simulator says and is the most reliable, since it doesn't rely on the power supply voltage being applied quickly to trigger it. I have had a quick play with Proteus simulation and the 555 timer and I also couldn't get it to work. The output voltage just went up to 1V. It must be a problem with the model, since it works in real life and LTSpice.