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time delay voltage pulse

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Master Mind

New Member
heyy guys.. i im baffled how to creat a a time delay voltage pulse ckt, that genrates a +5v voltage pulse after every 6 seconds (without using a 555 timer,or any timer at all) i want this for trafic light signal systemi.e my logic design project. I ve designed the rest of it. I waz thinking about using a diode clipper or sum sort of a RC ckt.
 

BrownOut

Banned
You've given no details on your restrictions. Yet you expect us to give you a solution, and bail you out. :rolleyes: Next time don't wait so late to ask, and give the information when requested.
 
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Sceadwian

Banned
How about this. Use a transistor as the main switch, at it's base use a series of diodes that will have a forward drop of about 5 volts. Do the math to find an RC time constant of 6 seconds to reach five volts and feed that into the base of the diodes that go to the transistor base. When voltage is applied to the base of the RC network it will charge to 5 volts, at which point the diodes will begin conducting triggering the transistor on. You'll have to chose your R value carefully though as it will need to limit current to the base to keep it from frying as well as still provide enough current to saturate the transistor.
 
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BrownOut

Banned
Program a PIC to output a pulse ever 6 sec. Technically, a PIC isn't a timer.
 
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Sceadwian

Banned
If you can use an opamp you're all set, they're easy to configure as a comparator. I played around with the transistor idea, but I couldn't find a low enough R value that would saturate the transistor without an absurdly large capacitance, the transistor was always in it's linear range and burning a lot of power, and in the real world itself up, you might be able to use two or more transistor in a darlington array to do it though.
 
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Sceadwian

Banned
I was bored so I played around with it in LTspice cause I don't generally create new circuits from scratch. It's pretty crude, but it works. The major problem being the turn off delay is the same as the turn on delay. Sorry about the size of the image, I didn't bother trying to make it small, was just a hack job. The blue line is the triggering input (in the shematic it's the pulsed voltage supply) and the green line is the load current. I used a 100ohm resistor as a sample load. The response will be much more square and less lossy in the transistors if the load is less. You could probably increase performance dramatically by changing the components used, perhaps to a true darlington. Just as a learning example I'd like comments from other users on how that circuit could be improved without adding significant complexity.
 

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Sceadwian

Banned
Just realized the diode is completely unneeded.
 

Sceadwian

Banned
I removed the gate resistor to the 2nd transistor, and put a 10k resistor between the collector and VCC and the power dissipating in the first transistor was completely eliminated. there are 60mw spikes on the second transistor during switching of the delay line and a 12mw constant load when it's on. Also 200k and 100u for the RC network seems to get closer to 6 seconds.
 
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