555 timer getting triggered on power up (2)

Thread Starter

Nixie64

Joined May 23, 2024
2
When I switch on the timer with 12v it starts immediately and pin 3 goes high. I don't like this ! After the first initial all work fine...
How can I suppress the first output on pin 3 after 12v switched on ? Thanks all...

Moderator edit: New thread created from this.
 

dl324

Joined Mar 30, 2015
17,096
Welcome to AAC!

When discussing circuits, you should post a schematic so we don't have to ask basic questions/guess.

For example, what is connected to the trigger and reset inputs?
 

crutschow

Joined Mar 14, 2008
34,817
I have 47uF from 12V+ to ground...Nothing changed....
That's because you didn't do what I suggested. :rolleyes:

The positive end of the timing capacitor goes to +12V, and the negative end to the 555 timing node that includes pins 2 and 6.
That way the capacitor initially sets the timing node to the plus voltage, which immediately triggers the THRS input and turns off the 555 output.
 
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Hymie

Joined Mar 30, 2018
1,293
My 555 timer cookbook advises to connect a large value capacitor (say 1µF) from pin 5 to 0V.
This has the effect of holding control voltage low (keeping the timer reset) while the supply voltage is applied.
 

dl324

Joined Mar 30, 2015
17,096
Schematic you posted in the other thread:
Schema aktuell V4.jpg
You shouldn't be having a problem with triggering on the first one shot at power on because you have a pull-up resistor on the trigger input. The second one should trigger because the output of the first will be low.

The symbol your schematic editor uses is terrible for conveying circuit intent. Your unnecessary white space, unnecessary (distracting) colors, and style make it even more difficult to read.

Your schematic is the second I've seen in as many days using clamp diodes on the trigger input. Signetics never suggested a clamp in their comprehensive application note in their 1979 Analog Applications databook.

You don't have a charging path for the timing capacitor on the second timer.

Why are you limiting the discharge current on the first one shot?

Why do you have R10 (10 ohms) in series with the relay coil?

Why do you have D4?
 

crutschow

Joined Mar 14, 2008
34,817
My 555 timer cookbook advises to connect a large value capacitor (say 1µF) from pin 5 to 0V.
According to my LTspice sim with a transistor-level 555 model, that works as long as the pin 5 time-constant is at least 3 times longer than the timing time-constant.
(The pin 5 equivalent resistance is about 3.3kΩ. which determines that node's time-constant.)
That could require a large capacitor if the timer is set for a long, high-level time period.

Connecting the timing capacitor com terminal to V+ should work without any added parts.
 
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Hymie

Joined Mar 30, 2018
1,293
My 555 timer cookbook offers a second method of ensuring that the circuit is not triggered due to supply initiation; and that is rather than connect pin 5 (reset) to the supply, use a resistor/capacitor to keep pin 5 lower than the supply voltage (as it powers up).

A 10nF capacitor connected from pin 5 to 0V combined with a 4k7 ohm resistor from the supply to pin 5 should work for most circuits (unless the power up is very slow).
 
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