# Calculating 555 timer periods

Discussion in 'General Electronics Chat' started by Dritech, Jun 12, 2013.

1. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi all,

I need to design a 555 timer with a high period of 1ms and a low period of 4 seconds. I used the site below as reference, but when using the formulas mentioned, I ended up with values which are not possible (either to large of with a negative value).
Can anyone tell me is the design on this site is good? or any reliable site that I can use to solve this?

http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/

2. ### bug13 Well-Known Member

Feb 13, 2012
1,210
38
Someone know more about the 555 timer correct me if I am wrong, but I don't think 4s low is practical for a 555 timer.

3. ### shortbus AAC Fanatic!

Sep 30, 2009
4,082
1,663
Did you see the bottom of that page? The note in blue type?

You could do what you want by using two 555 circuits or a 556(dual 555). In monostable mode. Set one for the high time that would then trigger the low time, then that would then trigger the first one after timing out. Repeat as necessary.

4. ### LDC3 Active Member

Apr 27, 2013
920
160
I can't get a 1 ms pulse, but I did get a 4 second low. Go to this web site and use the second calculator. I used 10R for R1, 1M for R2, and 5.8uFd for the capacitor.

5. ### LDC3 Active Member

Apr 27, 2013
920
160
The other option is to have a high for 4 seconds and a short low, and invert the signal.

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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A correctly orientated diode in parallel with R2 may do the job. R2 is bypassed by the diode in the capacitor charging phase and R1 determines the charging time. The discharge time is determined by R2. One would have to play with the design parameters to account for the diode voltage drop but this is a minor inconvenience.

7. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi again.

I will try the circuit referred by LDC3. What diode should I use for D3? and since a 5.8uF capacitor does not exist, can I change the resistors values to compensate for it?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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The attached schematic "worked" in simulation - no guarantee of real circuit result. Perhaps at least indicative.

I'd suggest using the CMOS version of the 555 timer and low leakage capacitor.

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9. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Rather than waste further effort on simulation [per my earlier post] I've built a working circuit based on the TLC555CP chip.

I don't have a means of measuring direct times for pulses but thought I'd try using a DMM to infer some results.

Rather than use a large capacitor to run at the very low rate (1/4 Hz) proposed, I scaled up the frequency to a value whereby I could obtain a steady mean value at the timer output pin 3.

Comment 1: As one might expect the real diode strongly influences the outcome and this is exacerbated at lower supply voltages.

The test of interest ...

Using a 12V DC supply with R1=330Ω, R2=820kΩ and a small signal silicon diode shunting R2, I obtain an output duty cycle of about 0.3% running at ~30Hz [measured on the DMM frequency range] with C=100nF. The goal would be a duty cycle an order of magnitude less than this: namely ~0.025%. When I try a lower R1 value (180Ω) to reduce the duty, the outcome is unfavorable with the duty actually increasing to about 0.43%.

I suspect this tells me that I am not going to be able to achieve the desired outcome using this approach.

It will be interesting to see if anyone else tries playing with this particular circuit and obtains the desired outcome.

10. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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If you use this circuit you can omit the second 555 timer, and use presets to give the desired time delays, for 1mSec use a 2.2K preset only set VR1 to 1K, for 4 Seconds use a 4M7 preset VR2 set to 3.7 meg, C1 use 1u Farad

spark8217 likes this.
11. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Nice one Dave - used slightly different values in actual circuit but works beautifully!

Dodgydave likes this.
12. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi,
Thanks a lot for the help. I did some changes and would like to form the waveform attached using two 555 timers (or a 556 IC). Is it possible to do this with accuracy? a slight delay will change the operation of the circuit, so it is not possible.

Just for you to know, these timers will be used for a circuit which will show revs/sec of a motor on two seven segment displays. The pulses at the top will be used to count the revs per second using a 4510 counter. The pulses at the bottom will be used to refresh the 7 segment every approx 4sec.

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13. ### shortbus AAC Fanatic!

Sep 30, 2009
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May be wrong here, but, 555 circuits are timers not clocks. They are not that accurate over a period of time. Shouldn't you be using a more accurate "clock" for your circuit?

14. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Are there other ICs which can be used as a clock and can produce such pulses please? (apart from micros)

15. ### WBahn Moderator

Mar 31, 2012
18,065
4,905
If you are trying to produce a 1ms pulse every 4s, then one simply possibility that gets you real close would be to use a 12-bit counter clocks at 1kHz. Using the Carry-Out bit would give you a 1ms pulse ever 4.096s. If you used a just slightly faster clock (namely 1.024kHz) you would just a pulse that is just less than 1ms every 4s.

For your clock, you can use a 32.768kHz oscillator, which is very cheap and very stable (it's intended for watch applications). You can use that to free-run a counter and use the 3rd bit to get a 1.024kHz squarewave.