555 Timer, automotive courtesy light

[drdewilliams]

I created a simple 555 one shot timer circuit activated by a momentary push button switch and connected to a string of LEDs under the dash to provide a 2 minute under dash light. The input voltage to the 555 is + 12VDC unswitched. However, everytime I impose a load on the circuit (like honking the horn) the 555 trigger is activated and the monostable pulse is generated. Seems like any variation in input voltage causes the timer to trigger. Do I need a smoothing capacitor across the 12V and ground and if so how much? Any help will be appreciated.

[MacFromOK]

A 100uF electrolytic should help, but it shouldn't be doing this.

Connecting to another power wire that isn't so heavily taxed should solve the problem, unless perhaps you've made an error in the circuit.

[CWB]

false triggering is most likely due to "noise" (spikes) on the B+ line .
yep , a cap might help (100uF electrolytic paralleled with a .1-.01 ceramic disk) .
making the trigger requirement a little "stiffer" will help out too .

[drdewilliams]

Thanks, I'll give it a try.

[Sch3mat1c]

Funny, my dad's initials are DEW. But he's not a Dr.