10 sec power on delay

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jms1989

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Hi, I'm looking to make a 10 second power on delay without the use of a micro controller. the idea is to apply power 10 secs after the ignition is turned on in a truck. It doesn't need to carry current, just enough to trigger a relay. I intend to use a 555 hopefully that triggers a transistor that then triggers the relay. How would you guys accomplish this?

This is for a radar detector so it doesn't go " welcome... welcome blah blah blah"
 
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Yes, a 555 will do the job. Its output may drive the relay directly if it does not require too much current.

This simulation shows how you will set it up:



Power is applied by closing the switch (shortly after the blue line). Ten seconds later the 555 output goes high, closing the relay.

After a few seconds, the switch was opened. The relay loses power. The capacitor takes a few seconds to discharge.

This layout is normally called a bounceless switch, if the delay were very short.
 

is that a ceramic cap? this sounds very doable, just gotta locate a 100uf cap. also, I'd want a diode across the relay to avoid back surge. it'll be powering a 12v automotive mechanical relay.
 

jms1989 said:
is that a ceramic cap? this sounds very doable, just gotta locate a 100uf cap.
Click to expand...

If you wish to use a lower Farad value, a 1 uF and 5.6 M ohm should work... provided the capacitor is in good condition, not leaky.

also, I'd want a diode across the relay to avoid back surge. it'll be powering a 12v automotive mechanical relay.
Click to expand...

Yes, you remembered what I forgot. Obviously you are up to speed about these things.

Revised schematic:

 

Ok, so I built my timer except I used a 47k control resistor and 100uf cap and it gives me about 10 seconds delay. It works great except now, I only get 5v from the output. Not near enough to trigger the relay. My question now is what would be good for it to switch to give me the source voltage I need for the relay? I tried a npn transistor but its only giving me ~3.5v. :/ what can I use to trigger my relay? I also measured the relay current to be about 140mA. I know the 555 can handle it but it lacks the voltage. Suggestions please sirs.
 

Something which I did not notice until now...

The resistor network at the Vin terminal affects the output V at pin 3.
By reducing the top resistor, it increases output V. There is a chance this will be sufficient for the 555 to drive the relay.

If it is not, here is the way to arrange an NPN so as to maximize current through a load. The key is to put the load in the collector leg.

 

Hi BradtheRed, please i tried your circuit in multisim but it is not working, i think, i could not understand its pins. would you please assign pin # in your circuit

Regards
 

I used Falstad's simulator. When it draws a 555 IC, it does not create the ground terminal (pin 1). It is 'assumed' to be connected.

This is not complete, electronically speaking. However conceptually it reduces clutter.

Did you draw the ground connection in your layout?
 



This circuit uses two BC817 SMD transistor and few other parts. Adjust needed time with capacitor.

Current consumption in delay state is 1,3mA.









33uF 2,18 sekunde
100uF 7,17 sekunde
470uF 33 sekunde
1000uF 1min 46sek
3300uF 5min 50sek




Best regards,
Peter

;-)
 

Thanks BradtheRed for your reply, yes i connected ground.

Would you please tell whether the below mention pins are correct

Your Pin
Name 555pin#

Vin = 8
dis = 7
th = 6
tr = 2
ctl = 5
out = 3

Please tell where pin # 4 is connected

Thanks tpetar for your post, it looks lovely, just on clarification can i use simple darlington.

regards
 
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sbaig22 said:
Thanks tpetar for your post, it looks lovely, just on clarification can i use simple darlington.

regards
Click to expand...

Yes you can. Circuit works nice and for your needs of 10sec you need to use capacitor around 140-150uF. You can use tantal capacitor.


Best regards,
Peter
 


Yes, those are correct pin assignments.

Pin 4 (Reset) is not needed in a bounceless switch role. I believe it should be connected to pin 8.

You can use the 555 IC method, or the simpler transistor-based circuit recommended by Tpetar.
 

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