I need 1 short micro switch closure to actually complete a circuit 2 times very quickly

Thread Starter

solver

Joined Sep 20, 2019
3
A small micro switch is mechanically activated and closes briefly as a quarter drops through a coin acceptor. The micro switch has only 2 wires and simply completes a circuit as input to the skill game's main processor board. I need the circuit to be completed twice quickly for each quarter instead of once. The 2 circuit closures must be done very rapidly so as not to conflict with the next quarter drop.

I have not yet been able to measure the micro switch terminals for voltage. I suspect a low (5-12) VDC.

I am a beginner at circuit design and have not been able to find an existing project kit or circuit design online. I have many game machines to modify, so I am hoping for a very simple and cheap solution.

Any help is greatly appreciated.
 

danadak

Joined Mar 10, 2018
3,577
Where does the second pulse go, to other logic or passive load ? I understand
the first pulse goes to motherboard as a logic signal, I presume its 0 or 5V signal ?
Is second signal also feeding logic ?

I can do it, and debounce the microswitch so you get two clean pulses
with a choice in both pulse width and delay between the two pulses with
fairly accurate stable timing....But you would have to build a simple jig
to program the 8 pin processor. Design would take a cap to bypass processor,
possibly one resistor, and the processor itself. It also would generate no glitches
on power up.

See attached.

Regards, Dana.
 

Attachments

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djsfantasi

Joined Apr 11, 2010
5,602
Look up dual edge detector circuits. They will produce one pulse when the microswitch closes and a second pulse when it opens. The microswitch output must be debounced.

This link contains several examples.
 

crutschow

Joined Mar 14, 2008
23,349
To properly design the circuit we need to know the approximate duration of the pulse generated by the micro-switch, and the desired duration of the two output pulses.
Do you have access to an oscilloscope to measure those?

If not, we can make a guess, but it may not work properly without some tweaking.
 

Thread Starter

solver

Joined Sep 20, 2019
3
Answers to questions above.

1) Both output pulses must go to the same place on the main processor board, just twice instead of once. The processor board counts the pulses (quarters) and then issues a credit accordingly. Essentially, I want each pulse to cost 12.5 cents instead of 25 cents.

2) The current duration of the micro switch pulse is simply one quick click as the coin travels by it and activates it via trip wire mechanically. I would guess 1/10 of a second or less. I cannot be any more specific than that.

3) The exact duration of each output pulse is probably not too critical, however the dollar validator (also feeds the main processor board) pulse is 50 ms long currently. I suspect that a pause of 50 ms between the 2 pulses would also suffice.

Again, I am a novice at circuit design, and unfortunately need a fairly specific schematic to follow.

Thanks to all of you for the help. I really appreciate it.
 

crutschow

Joined Mar 14, 2008
23,349
The simplest circuit would be a microprocessor as danadak suggested.

Otherwise, an XOR double pulse circuit is typically used, but if the input pulse is only 100ms, that will be problematic if you want two 50ms output pulses.

Below is the LTspice simulation of a circuit with an CD4070 XOR to generate the two trigger pulses for a 555 circuit which generates the 50ms pulses, that should work for you.
Edit: (Note that the circuit is not sensitive to any contact bounce from the micro switch, as shown by the simulated bounce at the first input pulse, yellow trace).

Vdd can be from 5V to 15V, but the trigger pulse from the switch must be the same voltage.

upload_2019-9-21_17-4-15.png
 
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crutschow

Joined Mar 14, 2008
23,349
With no R-C delay between U4 and U2, the propagation delay through U3 is only about 200 ns. Can a 555 trigger input respond that fast?
Good question.
That value is not specifically given in the 555 data sheet, but it shows the propagation delay from trigger to output is about 100ns (below), so I would think a 200ns pulse-width would be sufficient.
upload_2019-9-20_15-29-30.png
 
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Thread Starter

solver

Joined Sep 20, 2019
3
Thank you all for your expertise and valuable suggestions.

I will build the circuit in reply #7 and post the results when completed.
 

danadak

Joined Mar 10, 2018
3,577
The simplest circuit would be a microprocessor as danadak suggested.

Otherwise, an XOR double pulse circuit is typically used, but if the input pulse is only 100ms, that will be problematic if you want two 50ms output pulses.

Below is the LTspice simulation of a circuit with an CD4070 XOR to generate the two trigger pulses for a 555 circuit which generates the 50ms pulses, that should work for you.
(Note that the circuit is not sensitive to any contact bounce from the micro switch).

Vdd can be from 5V to 15V, but the trigger pulse from the switch must be the same voltage.

View attachment 186615
Crutschow would you mind posting the asc file for this, I would be grateful.

Regards, Dana.
 

crutschow

Joined Mar 14, 2008
23,349
Its looking for the model/symbol file for the 555, sorry to be a pest.
No problem.
You should be able to use any 555 model you have (in the Misc directory I think), but I can up-load mine if you don't have one.
 

danadak

Joined Mar 10, 2018
3,577
If I do a sim with the "misc" directory model, NE555, it looks like
trigger does not meet min specs ?


Regards, Dana.
 

crutschow

Joined Mar 14, 2008
23,349
it looks like
trigger does not meet min specs ?
What spec value?
If you are referring to a minimum trigger width, I haven't found a spec value for that (see post #12).

Below is my sim with the NE555 model:

upload_2019-9-21_13-49-25.png
 
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