# 555 stable timer circuit

Discussion in 'The Projects Forum' started by stevy123, Jan 24, 2008.

1. ### stevy123 Thread Starter Active Member

Nov 19, 2007
61
0
Hi All

this is probally a very simple circuit that i need to build around my 555 timer but am unsure of how to do it. I am looking to have a stable timer that gives an output for 1/4 of a second and off for 3/4 second. Can anyone tell me what i need to put around the circuit to give this result?

Many thanks
Stephen

2. ### hgmjr Retired Moderator

Jan 28, 2005
9,029
219
In the tradition of "give a member a fish, teach a member to fish" I offer this link to a good website for learning about all things 555-timer.

hgmjr

3. ### stevy123 Thread Starter Active Member

Nov 19, 2007
61
0
thank you for pointing me in the direction of that website, I was looking at the better timing circuit using the transistor. Would this be a good circuit to choose? Also what value would be suitable to use for the variable resistor to give a period of .25seconds on and .75 sconds off?

Regards
stephen

4. ### hgmjr Retired Moderator

Jan 28, 2005
9,029
219
Here is a link to an on-line calculator for choosing the component values for an astable 555 timer design.

The first calculator it for the monostable configuration and the second calculator is for the astable configuration.

Have a go at using it and let us know if you have problem getting it to work for you.

hgmjr

5. ### stevy123 Thread Starter Active Member

Nov 19, 2007
61
0

I have been working with the calculator that you gave me for the astable circuit.

i can get it to give me an on time of 0.69 seconds and an off time off 0.3 seconds by using
R1 = 500 ohm
R2 = 500 ohm

i have tried various other different combinations but cannot get it to be the other way round. i am looking to get an on time 0f 0.3 seconds and an off time of 0.7 seconds but cannot get this to work.

You any idea what size of components to use?

Stephen

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
There needed to be a diode added to the circuit diagram in order to get the ON duty cycle under 50%.

Also, your circuit would've used a LOT of power with those small values of resistance and huge capacitor.

See the attached schematic for the component values you need.

It was generated using 555 Timer Pro in the freeware mode. You can download the program here:
http://www.schematica.com/555_Timer_design/555_Timer_PRO.htm

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7. ### stevy123 Thread Starter Active Member

Nov 19, 2007
61
0
hi SgtWookie

Just to double check the sizes of the resistors

R1 = 360k
R2 = 1M

Will this circuit give me an on time of 0.3 seconds and off time of 0.7?

Many thanks
Stephen

8. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
OK, originally you said that you wanted it on for 1/4 second, and off for 3/4 seconds - which is what the schematic shows.

Is your requirement now changed to 3/10 seconds on, and 7/10 seconds off?
I've attached a schematic with updated values for that duty cycle.

Note that capacitors can vary quite a bit in their values; if it's marked 1uF 10%, it could be anywhere from 0.91uF to 1uF. In that case, the values of the resistors would need to be increased correspondingly.

Also, the schematic I previously posted calls for R1=360.7K and R2= 1.082M, not simply 360K and 1M. You can get those values for R1: 360K and 680 Ohm resistors in series (very close) and for R2: 1M and 82K Ohm resistors in series. Keep in mind that resistors have a tolerance as well, and they nearly always have a slightly lesser value than indicated. For example, a 10K, 10% tolerance resistor may be anywhere from just over 9.1K to 10K; you'll have to measure them to make certain.

You could use a smaller capacitor, but the resistors would have to be increased.

I suggest you download the free software I gave the link to in my prior post, and experiment with it. It's quite easy to use.

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