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555 timer power prob

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Spellman

New Member
i've made a timer cct with a 555 timer and its works ok but the problem is when i turn on the power. it activates the relay even though the switch from pin two hasn't made contact

could anybody help me with this prob it would be great
 

laroche73

New Member
555 output level

Is your load referenced to ground? The 555 output (pin 3) is driven high until the threshold comparator is triggered. You can either use a load connected to VCC or drive a NPN transistor from pin 3 to invert the output (use an external transistor for load currents over 200mA).

The following link has an animated display of the timers' operation in astable mode, and shows both VCC and ground referenced LEDs:
http://www.williamson-labs.com/555-tutorial.htm
 

k7elp60

Active Member
Timer circuit

I think the problem is some false triggering of pin 2. Try a 0.1uF capacitor in parallel with R2, it will keep pin to at Vcc on power up.
I think in the monostable mode and the circuit shown, pin three is low until pin 2 is grounded, then pin three stays high untill the capacitor on pin 6 and 7 charges to 2/3 Vcc, and then pin three goes low again
:D
 

Spellman

New Member
thanks k7elp60

i tried that and it seems to work ok but its hard to know cause it only happens the odd time
i'm all out of 1uF caps i just had the one
would it matter if i used a 10uF cap or if i used a 10nF cap is there any risk of damaging the timer by using these or which would be best
 

laroche73

New Member
false trigger on 555 powerup

You can also try this circuit: **broken link removed** It keeps the 555's internal flip/flop reset during power-up through an RC connected to the threshold input. And **broken link removed** shows another approach for reducing the trigger sensitivity of a 555 used in monostable mode.
 
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