Electronics > Beginners
A simple circuit to trigger relay for 30 seconds
sairfan1:
thanks a lot, that would be great help for a lay man
@Hero999 I was trying to understand circuit, what is transitor Q1 doing
thanks
Zero999:
--- Quote from: Jwillis on September 14, 2018, 07:07:50 pm ---I haven't tested this yet but I think it should work.You'll have to experiment with the transistor and base resistor.I made a guess on that. The solenoid can be omitted and the transistor can used as the switch instead.Should be a couple hundred milliamps at pin 3 available.You can also use a Mosfet like a BUZ 11 instead if you like,just omit the base resistor.If there are mistakes made I'm sure someone will point them out.
--- End quote ---
That will work, but it's an astable circuit, which will switch the relay/load on and off periodically. I'm not sure if that's what the original poster wants, which I believe is a monostable or one shot, i.e. push a button, the relay turns on for a certain length of time, then off again.
If can easily be modified for monostable operation.
EDIT 1: I've just realised something: 4700μF is far too larger capacitor for the 555 timer. The energy stored in it can easily blow up the discharge pin, when the transistor turns on! Reduce the capacitor value to 470μF and increase the timing resistor value to compensate!
EDIT 2: The 4700μF capacitor is only an issue in my monostable circuit. It's no problem in your astable circuit, as it discharges via a resistor.
--- Quote from: KMoffett on September 14, 2018, 07:38:23 pm ---I've used simple MOSFET/relay monostables.
Ken
--- End quote ---
Yes, that will work, just beware that it might shorten the life of the relay contacts a little, as they will not snap so sharply on, as the MOSFET will turn on very slowly, but it shouldn't be a problem if it's driving a solenoid directly. The time delay will also be heavily dependant on both the supply voltage and the MOSFET'd threshold, although that's a non-issue if the supply is tightly regulated and a potentiometer is used for the timing resistor.
Also note, as drawn the MOSFET will turn on, when power is initially applied. If this is undesired, the capacitor could be moved in parallel with R1.
You also might ask, why build the more complex circuit I posted, over the simpler one you posted? The advantages are: the delay is more predictable, less dependant on the supply voltage and completely independent of the MOSFET threshold voltage and the MOSFET turns on/off much faster. The delay also starts the moment the button is pressed, rather than released and has no bearing on hold long the button is held for, which might not be an advantage, just different. On the other hand it uses far more components, so could be considered to be overkill.
drussell:
--- Quote from: Hero999 on September 14, 2018, 10:18:26 pm ---EDIT: I've just realised something: 4700μF is far too larger capacitor for the 555 timer. The energy stored in it can easily blow up the discharge pin, when the transistor turns on!
--- End quote ---
If the discharge current would be too high on a very-long duration set-up with a large capacitor, couldn't you put a low-value resistor between the capacitor and the discharge pin?
I suppose I should look up the internal diagram of the 555 and re-acquaint myself with the circuitry, perhaps that won't work. :)
alsetalokin4017:
reasonable timing cap and resistor values can be chosen for 30 seconds on-time
trimpot can be used for timing resistor to provide adjustability
IRF3205 works fine without heatsink for reasonable loads, probably would not need relay at all in OP's application
tpowell1830:
--- Quote from: sairfan1 on September 13, 2018, 07:57:02 pm ---I have been deveoped some small projects using mcu, but this time i want a low cost wihtout mcu solution with this thought it will help me to learn electronics. as I'm not good in totally electronics based solution so need help.
I have a 12v 0.6A relay to lock a door, when i press a button it will activate the relay for 30 seconds and then it will release the relay again. I would like to have pot to increase or decrease little bit of time.
I have a little idea that i would need some transistors to turn relay on probabally BD series, some diaod and may be capacitor to hold relay for some time, some other approach could be using 555 timer.
Please advise whatever apprache you think is bettter.
thanking you,
--- End quote ---
How about a time delay relay?
https://www.grainger.com/product/5WML9?cm_mmc=PPC:+Google+PLA&s_kwcid=AL!2966!3!166591437759!!!g!81032003277!&ef_id=WndOVQAAAeEQiKRE:20180915023141:s&gclid=CjwKCAjwuO3cBRAyEiwAzOxKsudbn6D80FcSmPijKLBuVOM6wKUg4OA_i7uHnkohev6O68Y2zop9MhoCq7YQAvD_BwE
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