i have a device with a footswitch input. i want to build something that can switch it on an off randomly. the only problem is currently with a regular footswitch it takes two taps to go from off to on and vis versa. would it be possible to build something to do this with 555s?

thanks!

 

Wouldn't any "random" string of pulses do that? The fact that it takes 2 pulses to change state is irrelevant.

The big problem is to make the 555 give a "random" pulse. Do you mean truly random, or more likely, not by any obvious pattern?

Assuming you are limited to a 555 and cannot use other devices, one thought would be to add a highly variable discharge or charge path to the capacitor (e.g., a large conductive plate). The rate of discharge would be affected in that example by variables of time, humidity, temperature, other objects, and anything else that could charge or discharge static electricity from the conductive plate.

John

 

well, yes, but i figured that would lend itself to longer intervals by nature, but maybe that's not really a concern.

i'm not looking for true randomness (if such a thing exists) but the appearance of it. i'm not limited to 555s per se, but i do have some on hand so i thought i'd ask.

the conductive plate idea sounds very interesting! i like the idea of it being affected by the environment.

 

If you search on capacitative sensors, you will find a wealth of information. Another option would be a simple RF receiver without any sort of code filter. Turn up the sensitivity. You will get lots of false signals without any apparent pattern. Some of the hobbyist, non-coded garage door receivers (300 MHz range) might be useful for that. John

 

very interesting ideas john, thanks. could you point me in the direction of a circuit that this technique could work with? i'm at the stage where i can build most things i want to, but i don't have the technical skills to design. thanks!

 

Check the datasheet for the 555. It and this site http://www.uoguelph.ca/~antoon/circ/circuits.htm have plenty of circuits. Here is another example of a monostable pulse generator from this site: http://home.cogeco.ca/~rpaisley4/LM555.html#2

John

 

thanks for the circuit. say i'm going to go with the conductive plate idea. how large should it be? what is the best material? where does it go in the circuit you attached? between C1 and pin 6?

thanks

 

In thinking about your question a bit more, I think that a multivibrator (oscillator) with the output driving an LED would be a good place to start. That is, copy one of the circuits for a blinking LED. Try to make it oscillate slowly (about twice as fast as you want the final circuit to switch on and off), using large resistors and a capacitor (just guessing) of about 1 uF. You will want to use the largest resistors and smallest capacitor with which the circuit will still work. Then add a large plate to the positive side of the capacitor (6 to 8" square, maybe a tin can?). See if you can change the frequency of the LED blinking with your hand etc. near the plate It will take some experimentation. I have never done it.

There are other ways to get the inconsistent signal, so don't get discouraged if this doesn't work.

John

 

thanks a lot! this is a very cool idea. off to the work bench with me then.