Hi, I've made my niece a BMX start gate to practice on. Currently it works on a 12vDC 555 monostable timer, which at present releases the gate after 20seconds or so, Is there any way I can modify or add so it works randomly from 20secs to 30secs. Any help or advice would be appreciated.

 

post a schematic of the circuit you are using to get help. Usually, by changing a value of one or 2 components on the circuit will vary the time delay..... so If you had a circuit to post maybe someone could incorporate a potentiometer to change the timing...

B. Morse

 

Sorry, should have attached that

 

The "random" aspect of what you're trying to do makes it pretty difficult with simple techniques. Could you program a microcontroller? I could imagine pushing a "start" switch where the duration of the button press was used to start a pseudo-random number generator (so it would be different every time: if you're counting microseconds, there are a lot of possibilities within just a tenth of a second). Then the actual interval would be 20 seconds plus some random amount between 0 and 10. But using analog components or logic chips, I wouldn't know how to do it.

Well OK, there is a way to produce pseudo-random numbers using a shift register. You could use the button-down time plus a fast clock to set an initial value, then use that as a timer interval. But I can see lots of components being needed for this.

http://en.wikipedia.org/wiki/Linear_feedback_shift_register

 

How about adding some sort of environmental input, temp wind, light ...
If you had a min time > look for some delta > max time you could guarantee it would always trigger but it would be hard to know when.

The other thing you could do is time based on hard / long the button is pressed.

Just musing
Al

 

The "random" aspect of what you're trying to do makes it pretty difficult with simple techniques. Could you program a microcontroller? I could imagine pushing a "start" switch where the duration of the button press was used to start a pseudo-random number generator (so it would be different every time: if you're counting microseconds, there are a lot of possibilities within just a tenth of a second). Then the actual interval would be 20 seconds plus some random amount between 0 and 10. But using analog components or logic chips, I wouldn't know how to do it.

Well OK, there is a way to produce pseudo-random numbers using a shift register. You could use the button-down time plus a fast clock to set an initial value, then use that as a timer interval. But I can see lots of components being needed for this.

No not able to program a microcontroller, would have to be one bought over the counter, Had thought of trying to incorporate a pause in the timing, i suppose the easy way of doing it is to attach a button to the reset, but that would need someone to press it.

 

How about adding some sort of environmental input, temp wind, light ...
If you had a min time > look for some delta > max time you could guarantee it would always trigger but it would be hard to know when.

The other thing you could do is time based on hard / long the button is pressed.

Just musing
Al

Had'nt considered enviromental input, using a sensor, but not sure on set up would probably need setting up each time its used due to different conditions.

 

If you look for a rate of change threathold rather than a spaciffiv level then setup wouldnt be a problem.
For example a tiny beed thermistor or those hard-drive temp sensors that Maplin sell have such a low thermal mas that they constantly change unless you mount them properly.

expose one of these to a slight breese or put your hand near it to see the effect.
http://www.maplin.co.uk/Module.aspx?ModuleNo=98838

If you feed the output to an opamp with a huge gain, ac couppled to get rid of any bias, all you would see is spikes coresponding to changes.

Might be worth fiddling about with.

Same sort of idea ... temp sensor on the button face again you have the variables finger temp and time toutched to play with. compare top face with one burried a little deaper perhaps, again self compensating for slow environmental changes.

I am sure you can get psudo random without getting complicated digital circuits involved.

Al

 

Still thimking sorry .....
In fact sack the button altogether:-
Two sensors bonded either side of of som relativly poor thermal conductor, your plastic box lid perhaps.
Electrically arrange them as two legs of a full bridge connected to a n amp with a high gain.

Touch the top sensor > bridge goes imediatly out of equilibriam and you arm the release circuit (buzz light something as indicator). > finger removed and so relativly random time later the bridge equilises again and you fire the release output.

I would love to see somone predict that timing.

Sorry for the two posts but the implimentation didnt occur to me until after I submitted the first one.

Al

 

If you do not mind a few ICs look at the verry bottom of this post-Similar threads- Random timer by nottechnical, esp. pages 15 on. Just change timing to suit.

 

I think I will have to try all the ideas to see which will work out better, the sensor could be interesting, Thanks everyone for your replies.