# Velleman MK111 555 Astable Operation

Discussion in 'General Electronics Chat' started by elec_mech, Jan 30, 2013.

1. ### elec_mech Thread Starter Senior Member

Nov 12, 2008
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Below is the schematic for the Velleman MK111. The kit claims the off (pause) time can be set between 2.5 to 60 seconds, while the on (pulse) time can be set between 0.5 to 5 seconds.

According to my limited knowledge and research, the following equations are used to calculate the on and off time:

ON (Pulse) Time = 0.7 x (RV2 + R1) x C3

OFF (Pause) Time = 0.7 x (RV1 + RV2) x C3

Thus,

ON_min = 0.7 x (0 + 1000) x 0.0001 = 0.07 seconds

ON_max = 0.7 x (47000 + 1000) x 0.0001 = 3.36 seconds

OFF_min = 0.7 x (0 + 1000) x 0.0001 = 0.07 seconds

OFF_max = 0.7 x (1000000 + 1000) x 0.0001 = 70 seconds

Hence, I get an ON range of 0.07 - 3.36 seconds, not 0.5 - 5 seconds and an OFF range of 0.07 - 70 seconds, not 2.5 to 60 seconds.

I'm trying to help someone else on AAC use this to get an ON and OFF time of 1 second each. The values are reasonably close, but the 2.5 second minimum pause (off) time is bugging me since it so far off from the calculated values. Am I using the wrong equations or did I put a decimal place in the wrong spot somewhere?

Any help is appreciated.

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2. ### elec_mech Thread Starter Senior Member

Nov 12, 2008
1,513
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Well, I built the circuit and tested it. It is possible to reduce the pause time to less than 2.5 seconds. Because a single-turn 1MΩ pot was used, I assume the 2.5 second minimum was listed because the tuning isn't very precise, especially at the extremes of the pot.

I did not think or have the time to test the full timing range of the pots, perhaps I'll do that once I upgrade my scope.

3. ### tracecom AAC Fanatic!

Apr 16, 2010
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1,396
I breadboarded the timer per the schematic, except I used a 50k instead of a 47k for RV2.

With the pulse duration set to minimum, I could get a range of ≈2 pulses per second to 1 pulse every 72 seconds. If the pulse duration is set higher, then the maximum rate is reduced.

It is possible to get about 1 second on and 1 second off, but adjusting the pots is touchy.

Last edited: Feb 1, 2013
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4. ### tracecom AAC Fanatic!

Apr 16, 2010
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If you don't need the adjustability, the circuit attached will provide very close to what you are seeking with no touchy pot turning. With a little effort, it could likely be adapted and built on the MK111 circuit board.

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Nov 12, 2008
1,513
193