Toggle switch to push button switch conversion (Arduino or IC logic)

Question

I would like to build an interface for a flight simulator consisting on various toggle and trigger switches, unfortunately i can't just simply flip a switch to an ON state and leave the signal like that, i need to send a quick pulse every time you toggle the switch ON or OFF, as if you where to push a button on a keyboard to send that command.

In simple words... i need a toggle switch that will produce one pulse when you flip the toggle switch ON. And a second pulse will be generated when the same toggle switch is flipped to OFF. Here is a GIF to better illustrate that... http://www.desktopaviator.com/Products/Model_2120/2120B.gif

If there is a simpler way of doing this without using an Arduino, like an IC or a special toggle swich, I would like to know, otherwise, I plan on using an Arduino to do this.

Thank you for your time!

jw0752 · Accepted Answer

Hi Silviu; I have drawn a schematic and added some notes. . Don't worry about right answers as everyone contributes good ideas. If you have clarifying questions let me know. John

shabaz · Answer

Hi Silviu,

An easy method to produce pulses is to get a 'quad XOR gate' ic.

Connecting one input of a gate to 0V makes a simple buffer which does nothing except delay a signal slightly. Do this with three of the gates so that you have 3 buffers. Connect them up in series. Lets call them gates #1,2,3.

Feed the final output from the third buffer (gate #3) into an input on the fourth gate. Connect your switch to 0V and (say) a 47k resistor to your supply rail. Connect the switch/resistor junction to the input of the fourth gate and the remaining input on gate#1.

The end result is that you get a brief pulse (a few tens of nanosec) whenever the switch is opened or closed.

But, I'm not sure this is what you really want. If (say) the power is off, and someone changes the toggle position and then powers on, then your software will have no idea what the initial toggle position is. Or, if the software is polling instead of using an interrupt, then it may skip some pulses (Maybe this is acceptable - depends on the actual application).

For these reasons it would be best to try to design it so you can accept a normal toggle input, rather than a pulse if possible.

shabaz · Answer

Hi Silviu, If you want to do it using an Arduino alone, you will need 80 pins for 40 switches and 40 outputs, so that may be unfeasible. Anyway, below is some pseudocode that will do it for (say) 4 switches and 4 outputs. It is pseudocode since I'm not totally familiar with Arduino and won't compile, but it should give you ideas on how to arrange your code. It's not particularly efficient code, but was a quick thing. The logic should hopefully make sense, so that you can translate it into code. // assume inputs are numbered 0-3
// assume outputs are numbered 4-7

set_outputs_low()
{
 // this function sets all four outputs low
 int k;
 for (k=4; k<8; k++)
 {
 digitalWrite(k, LOW);
 }
}

main()
{
 int i;
 char newval;
 char togglestate[4];
 
 // set all outputs initially low
 set_outputs_low();

// read the initial state of the 4 toggle switches connected to
 // inputs 0-3
 for (i=0; i<4; i++)
 {
 togglestate[i]=digitalRead[i];
 }
 
 while(1) // loop forever
 {
 // For all inputs, set an output high if a corresponding input has flipped
 for (i=0; i<4; i++)
 {
 newval=digitalRead[i];
 if (togglestate[i] <> newval) // has the state changed?
 {
 togglestate[i]=newval;
 digitalWrite(i+4, HIGH); // set the corresponding output high
 }
 }
 
 // now sleep for half a second
 sleep(0.5);
 
 // now just set all outputs low
 set_outputs_low();
 } // loop forever
 
} It sets an output high if it thinks the corresponding toggle input has changed from the previous value, and blindly sets all outputs low every half second, which has the desired end effect. If you want to scale to 40 inputs and 40 outputs with an Arduino then some form of I/O expansion is needed. This makes the circuit more complex and larger of course - things like PCF8574 can each handle 8 I/O, but there are surface mount devices that can do 16 I/O, or maybe higher.

R_Phoenix · Answer

You could also just use a MOM-OFF-MOM toggle. That is a Momentary toggle ON and OFF that returns to a center position. DPDT would work but it only needs to be a SPDT and therefore a bit cheaper. If you use the center pin as your output, you could send a positive for ON and a ground for OFF (as an example) or both positive or both ground.

billpenner · Answer

adding to the comments by shabaz ; The following might of help. Since you are using many switches and apparently need them to be independent, you could put all the resistors,capacitors and ICs on a PC board to simplify wiring. The ICs are quad XOR gates so each IC will handle four switches. The pulse width can be adjusted with R8 and C3. Using the values shown will produce about 1/2 second. The capacitors can be ceramic, which is very small. The ICs are available in both the standard dual inline and small form factor. I don't know what you are going to drive with the output, so I am not sure if the IC will handle the necessary current. Could you let us know where the output of this conversion goes? Bill

jw0752 · Answer

Hi Silviu, I see that you have marked this question closed but I thought I would throw an old fashioned idea at you anyway. If you are able to use DPDT toggles you could hook them up to reverse the polarity of a capacitor. In this way each time the switch is toggled the capacitor would discharge and recharge creating a current of duration depending on the size of the capacitor. I prototyped a small circuit and was able to get an LED to turn on for one second each time the switch was toggled. If this idea fits your need you could save lots of hardware and programming. If you would like a schematic for the little prototype I put together let me know. John

billpenner · Answer

Hi Bill Again.. To add an indicator to previous post, just add a resistor and a LED. see revised diagram. PS: This also applies to Chaim Smoller's question.

jw0752 · Answer

Hi Silviu, Actually my design is for a standard DPDT toggle switch with only side to side, no center off. I call the one you described DPDTCO. Once the switch is flipped from one side to the other the current flows as it charges the capacitor. Since we are dealing with direct current the flow stops as soon as the capacitor is charged up. The DPDT switch, if flipped to the other side, will reconncect the capacitor to the circuit reversed. This will allow the charge that is in the capacitor to exit and a new reversed charge to flow into it. I think the use of a center off switch would not work as well as the charge would have time to leak out while the switch is in the center position. While you would get a pulse each time you switched from the center position it would only be half the duration of using the switch the way I described it. As far a the actual style of the DPDT switch it can be any kind from a rocker to a slide. One more note, in my original post I mentioned that point "one" could be used to determine the up or down position of the toggle. The sensing of this point must be high impedance or you will be putting a resistance accross the capacitor when the switch is in one position. John

D_Hersey · Answer

Do not use this technique unless the logic family you are using publishes a minimum propagation delay!

(NTBL)

http://www.fairchildsemi.com/an/AN/AN-118.pdf

RCA published an AN entitled 'CMOS multivibrators' or some-such which explains this.

Former Member · Answer

Hi The stop switch is nothing more than a big slide switch which uses an attached bar magnet sliding in close proximity to a enclosed reed switch. The switch does nothing more than to supply 12 or so volts dc to an input board which eventually will turn on the mechanics and electronics of a pipe chest. My problem is we have a sound module where the stops are turned on and off by a momentary push button contact. There were external terminals added to do the same function by remote wiring. The late organs are control the stops or voices by this method even with these stop knobs. But they are nothing more than momentary contact switchs which activate when the knob is pulled or deactivated when the knob is pushed, and is on a spring return with an indicator lamp inside. We want to control this box with the original stop mechanisms on the organ console. So far I cannot figure a circuit which would be able to give me 2 states--an on state and an off state. The on state is easy, it is when the voltage is removed how does that get translated to a pulse to trigger the box to shut the stop off? The pulse width can be 500ms

Robert Peter Oakes · Answer

so referring to the question regarding using a simple reed switch, why cant you do this or something like it when switch is made, then the power is applied through the LED which turns on, you get a pulse while cap charges when you release reed, power is removed, LED and resistor discharges quicker then the leg with Cap and therefore you get another pulse you may need to play with values to get it to the right timing but this or a variation of it should work ? you could even remove the top resistor, not sure why it is needed anyway

phoenixcomm · Answer

my G-d you guys like to work.. if you are going to do a de-bounce circuit, please use something that was designed for this job!! dont rebuild the darn world!! you should look at the lowly 74xx14 http://www.ti.com/lit/ds/symlink/sn74hc14.pdf this is Schmidt trigger.. you can also use a 555 but that cost more!! ~~ Cris H.

Former Member · Answer

Hi As I mentioned, we examined the stop switch and found there are provisions to mount a reed switch midtravel, so all that is necessary is to add a pair of wires from each stop switch to its respective terminal pair on the sound module. But in the process I have been able to tap some great minds. I thank all of you for that. IT may be better to go this route as the process is easily reversible when real pipes are installed, and the original stop and driver circuit is still intact. But we will check out the devices and circuits to give our budding technician and BSEE graduate more in-depth knowledge for future application. Chaim

Former Member · Answer

Thanks all. The circuit works fine with added reed switch for off sense. The indicator LED loads the circuit and is not needed and upsets the logic anyway. We could not find a simple satisfactory system where the stop is pulled high--sending a pulse and when pushed to 0 producing another pulse. The output is connected to an opto-isolator which briefly pulses the sound module box. WE notice also when the circuit is depowered and repowered the circuit will always revert to "off" position.

Toggle switch to push button switch conversion (Arduino or IC logic)

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