# How to reverse polarity on every 2. pulse?

Discussion in 'The Projects Forum' started by dsk, Jun 16, 2013.

1. ### dsk Thread Starter New Member

Feb 24, 2013
15
2
Hi.
I have a clock, delivering + 24V signal for about 1.5 second every minute.
(max 0.2 A)
I would like to run an old slave clock (classroom clock), but this one wants every 2. pulse of reversed polarity.
Whats the most easy and lo-cost way to make or buy a stable solution for this.

Summary: input +24 V for 1.5 sec evry minute.
Want output +18-24V (1-3 sec) >=75 mA every 2. min
and -18-24V (1-3 sec) >=75 mA every 2. min.
The circuit should contain some kind of short circuit protection, resistor, fuse.

Thank you for all hints!
dsk

2. ### GopherT AAC Fanatic!

Nov 23, 2012
5,980
3,707
Do you need the negative pulse at the same time as the positive pulse or do they trigger on alternate minutes?

What power supply do you have (access to neg voltage or will you need a chip to provide that)?

3. ### dsk Thread Starter New Member

Feb 24, 2013
15
2
I have a dc 24 V switch mode power. (2 wires out, 0 and +24V) I'm so oldfashioned so I was thinking relay switching the wires .

dsk

4. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
If it's a (really) old clock, that's probably exactly how it was done. Or a clock motor and contacts.

5. ### dsk Thread Starter New Member

Feb 24, 2013
15
2
The orignal master clock was a pendulum clock with 2 separate mercury switches giving egither a positive or negative pulse one (+) every every odd minute, and one (-) every even minute.

Later master clocks used mechanical contacts or even relays. My clock gives the same signal every minute and are made for an another system.

dsk

6. ### #12 Expert

Nov 30, 2010
16,248
6,744
Here is a first attempt at a block diagram. Includes a suspect for the negative supply chip.

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7. ### dsk Thread Starter New Member

Feb 24, 2013
15
2
The block diagram seems to explain a solution, but the enclosed circuit is a little over my head, I kind of fell off at the time when the telpehones got push-buttons .

dsk

8. ### crutschow Expert

Mar 14, 2008
12,977
3,220
Below is a circuit that should do what you want. It consists of a flip-flop controlling an H-bridge to alternate the direction of the voltage across the clock (R_Clock) from the clock pulse source (V_Clk) for each pulse. The Schmidt trigger inverter provides switch de-bounce function as well as inverting the clock so that the FF changes state at the end of the pulse.

The MOSFETS can be just about any small devices.

Note that the external supply voltage is 12V, not 24V (the CMOS circuits have a max voltage rating of 18V) but it only has to provide less than a mA of current. This can be done with a small wall-wort supply. Alternately eight AA batteries would power the CMOS circuit for well over two years.

The supply and ground must be connected to the CD40106B inverter as well.

Make sure that all unused pins on the CMOS circuits are connected to ground (common).

You can add a fuse in series with the V_Clk source for safety if you like.

Last edited: Jun 17, 2013
9. ### dsk Thread Starter New Member

Feb 24, 2013
15
2
Looks great Thank you!
Ill guess I just my use 2 equal resistors to a make a bridge with approx 12V in the middle.

Could be a nice project, dou you have any internet shop to recommand for theese parts? ( Of couse no no blame on you if it doesent work for me here in Norway.)

dsk

10. ### crutschow Expert

Mar 14, 2008
12,977
3,220
Yes, you can use a two resistor attenuator (say 1kΩ each) to make 12V. Just be sure you decouple the 12V to ground with a 10μF (or so) in parallel with a 0.1μF.

Malpin in the UK may have them. In the US, Digikey, Jameco, or Mouser are likely sources. You might also try Ebay. You can always try a Google search.

11. ### ifiaxxy New Member

Jun 5, 2014
10
0
Hi, I am trying to build a circuit to reverse the polarity of a pulse signal. I have used the circuit shown, but after I got every part in the breadboard, I found that the current across R_clock in your diagram did not provide the result shown in the software. Instead, the current behaved like a regular pulse. I have also tested the voltage: the voltage at the left of R_clock is about the same as the input signal and that at the right of R_clock is about to be zero. My supply voltage is 12V and I have grounded all the unused pins of the flip-flop and the Schmitt trigger (I use cd40106BCM instead of cd40106B though).
As the circuit is not working, I'm not sure why and I'm hoping you can help me figure it out.
Below is the design of my circuit, everything is the same except I've added op-amps to get an output voltage whose pulse has reversed polarity
Thanks

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