# 555 question

Discussion in 'General Electronics Chat' started by snaker5, Jan 28, 2010.

1. ### snaker5 Thread Starter New Member

Jan 26, 2010
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0
I have a 555 which is designed to hold the signal high for 15 seconds. When I supply power it goes high for 25 seconds and the goes low. After it goes high it holds it high for the 15 seconds. Why does it hold for 25 seconds in the beginning?

Thanks

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
It's due to the initial charge time for C1.

When first powered up, C1 is completely discharged.

The threshold and trigger points are at 1/3 and 2/3 Vcc. If the cap has to start charging at 0v, the cycle will be longer.

One way to fix the problem is to take the C1 value, and divide it by three.
Then connect 2/3 of that value to ground, and the other 1/3 to Vcc.
That way, when you first apply power to the circuit, the trigger/threshold inputs will be at 1/3 Vcc, and the timing will then be consistent.

3. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
A 555 will switch, by design, when the voltage of the timing capacitor is at 1/3 or 2/3 that of the Vcc.

You have selected timing capacitor and resistor such that it takes 15 seconds for it to charge from 1/3 to 2/3 of VCC. When you power ON the circuit, the capacitor is NOT charged and therefore has to be charged from 0V to 2/3 of VCC so it will takes 50% more time.

There are "tricks" to charge the timing capacitor to 1/3 VCC immediately upon power ON so you will get the correct timing of 15 seconds.

4. ### snaker5 Thread Starter New Member

Jan 26, 2010
5
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Thanks guys. Also is there anyway to have the 555 start at low for 15 seconds when power is applied to it?

5. ### thyristor Active Member

Dec 27, 2009
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Connect pin 4 (reset) to Vcc through a 100K resistor (approx) and also to ground through a 100uF capacitor. When the circuit powers up, the capacitor will hold pin 4 low (thus the output will be low) until the capacitor charges up sufficiently to make pin 4 high and the astable starts up.

You will have to play around with the values to get it to exactly 15 secs but the principle holds.

6. ### snaker5 Thread Starter New Member

Jan 26, 2010
5
0
I got this to work, thanks. What is the best way to discharge the capacitor after the 555 goes high?

thanks

7. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You need to start posting schematics on how your circuit is currently configured.

If you want to discharge the pin 4 cap immediately when pin 3 goes high, connect the collector of an NPN transistor to pin 4 via a resistor (call it R4), emitter to ground, base to pin 3 using a 1k resistor. Experiment with different values of R4 to get the desired pulsewidth.

To make this whole process much more simple, connect C1 to Vcc instead of ground. That way, C1 is initially charged instead of initially discharged.