I'm a retired electronics engineer and keep my hand in by maintaining a small electronics workshop with a basic 'store cupboard' of components and the basic range of test equipment, power supplies, 'scopes, sig gens and so on.
I was recently asked to design and build an unusual circuit and though I've done it, and it works, I just get the feeling that there's a more elegant way to do it than my solution, so I thought I'd sign up here and pop the question.
The request was for a time delayed switch. The user wanted to be able to set a delay time between 10 seconds and one minute, press a momentary button switch and after the set delay time had elapsed, have a normally open output switch close for no more than half a second then open again, after which the circuit must return rapidly to it's quiescent state and be ready for another push button trigger within a few seconds. The unit had to be self contained, robust, easily transportable and its output switch had to be rated at 5A minimum.
The easy bit was the time delay, and in this case I used an LM741 in comparator mode with a simple RC network to the non inverting input to provide the variable time delay and with its output switching a transistor to energise a 6v/70R relay with 7A contacts which provided the output the user required. The tricky bit was to have that relay energise for only ½ a second, then get the circuit returned to its quiescent state with the LM741 non inverting input back to low within 'a few seconds'
Without going into detail I managed this with a second DPDT relay which latches the momentary button switch when energised and discharges the LM741 timing capacitor when deenergised. This second relay is held energised for the half second required by another RC network. This works OK and I can get it all in a small ABS box with a PP9 battery but as I say, I just have a feeling that there's a more elegant way to do it than messing around with a second relay and RC network - but I'm darned if can come up with one !! So before I go ahead and etch the board and build it for the user, (it's on the breadboard at the moment), I thought I'd see if anyone had any ideas on how else this requirement might be met.
Thanks for your interest.
I was recently asked to design and build an unusual circuit and though I've done it, and it works, I just get the feeling that there's a more elegant way to do it than my solution, so I thought I'd sign up here and pop the question.
The request was for a time delayed switch. The user wanted to be able to set a delay time between 10 seconds and one minute, press a momentary button switch and after the set delay time had elapsed, have a normally open output switch close for no more than half a second then open again, after which the circuit must return rapidly to it's quiescent state and be ready for another push button trigger within a few seconds. The unit had to be self contained, robust, easily transportable and its output switch had to be rated at 5A minimum.
The easy bit was the time delay, and in this case I used an LM741 in comparator mode with a simple RC network to the non inverting input to provide the variable time delay and with its output switching a transistor to energise a 6v/70R relay with 7A contacts which provided the output the user required. The tricky bit was to have that relay energise for only ½ a second, then get the circuit returned to its quiescent state with the LM741 non inverting input back to low within 'a few seconds'
Without going into detail I managed this with a second DPDT relay which latches the momentary button switch when energised and discharges the LM741 timing capacitor when deenergised. This second relay is held energised for the half second required by another RC network. This works OK and I can get it all in a small ABS box with a PP9 battery but as I say, I just have a feeling that there's a more elegant way to do it than messing around with a second relay and RC network - but I'm darned if can come up with one !! So before I go ahead and etch the board and build it for the user, (it's on the breadboard at the moment), I thought I'd see if anyone had any ideas on how else this requirement might be met.
Thanks for your interest.
