Trigger a monostable 555 timer at power up - every time!

555 timer monostable circuit

Hi

I need a small gadget for the car, where each time its powered on it lights a light for about 5 secs, then turns it off.

I've robbed a Maplin circuit, and moved the 10 nF cap from pin 5 to ground, to pin 2 to ground, and now the circuit sometimes triggers on power up but not always.

I'm wondering if the cap is maybe staying charged, and putting something like a 1M resistor across it would do the trick?

Sounds like a horrible bodge, I'm open to suggestions on how to do this properly.

Many thanks

nige

monostable 555 timer circuit

You really need to post your schematic. No one is going to know the exact Maplin circuit you "robbed".

Take a look at:

https://www.technologystudent.com/elec1/5553.htm

This is the basic 555 monostable circuit. One problem is charge stuck in C1. If you close the switch for several seconds, open the switch and then close it again quickly, a second pulse may not be produced. One solution is to connect a diode across R1. The anode of the diode goes to the R1 and C1 junction. When the switch is open, the diode bleeds off the charge in C1 and dissipates it into the power pins 4 and 8. Also watch the choices for R1 and C1. You want to use the biggest resistance and smallest capacitor that will give the required time delay. This is to minimize the storage capability of C1. Of course, if you may R1 too large, then the diode's leakage current becomes significant and you will find the time delay to be smaller than expected.

555 monostable circuit

Hi

Regards the schematic, that's a fair point, sorry.

Its the Maplin's 555 Monostable Switch project, that I thought I might adapt to suit my needs, but it didn't come with a schematic, and like an idiot I didn't draw one.
(I can demount the circuit, but it takes a while and I can't do that until this evening. Then, of course, the shops are shut, so I can't go and buy the bits I need! So my preferred way is to buy a bag of bits that should cover what I need, so I don't get part way again like I did last night).

I tested it out in the house on a 12V battery and it worked fine, but now its hooked up in the car it only triggers at power-up if its been left off for a long while.

It has a switch to trigger the 555, and when pressed that still produces the required "on" period.

Googling, I find many solutions to the problem of unwanted triggering at power-on, struggling to find how to force it to happen!

I'll give the diode a try this evening, I assume any low value signal diode will do?

Equally, I can junk this circuit and start again, if there's a better way of doing this - doesn't have to be a 555. Need to light a small bulb for ~ 5 secs when power is applied.

Thanks for the help, and sorry to be so sparce with info.

Nige

monostable 555 timer

Small signal diode is fine. 1N914, 1N4148, etc. You can also use a larger rectifier like 1N4001 if that is what is handy.
Since you are running from a car battery and do not need extreme low power, you could also add a bleed resistor directly across the power pins of the 555. This gives yet another path for the capacitor's charge to dissipate in. Something like 4.7K or 10K.

555 monostable

Hi

In fact I replaced the trigger switch by a 0.1uF cap and a 1M resistor in parallel, works a treat.

Thanks for all the help/support, really appreciated.

Nige