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Confused by monostable setup

Hi guys. Once again, I am trying to get my head around an odd layout, and having trouble.

The only experience I have with a monostable circuit is using a 555 back in school. However, I'm not sure if that's the right tool for the job here, as as far as I know you need a low to act as the trigger.

If somebody could please recommend a solution to this task, or (if you've really got lots of time) to supply me with a simple circuit diagram, would be much appreciated.

I have 2 power supplies from external circuits - one provides 5V DC and is always on, the other provides 12V DC and is switched externally. I also have several DPST relays connected to various external components (the circuits controlled by the relays should have no electrical connection to this one, so what they are is probably irrelevant).

When the 12V input is 'on', both relays should be closed. When the power goes off, all the relays except R1 should open. R1 should open after 200 seconds, unless the power comes back on during that time (in which case the whole thing starts again).

Can anyone suggest how I would go about setting this up?

Reply to
Somebody Else's Wossname
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A counter that gets reset when the 12V is ON. Start counting when the 12V goes OFF. When the count gets to 200 seconds, it freezes, and R1 opens. If the 12V comes back before 200 seconds, the counter resets and R1 stays on. Easiest way to do this is with an 8 bit counter. Look for the most significant bit to go to a 1. You need a frequency of 200/128 = 1.56 Hertz. You could have an astable running at that frequency, or take 60 Hz and divide it by 94 (probably 100 would be OK).

Tam

Reply to
Tam/WB2TT

What you want is a retriggerable monostable. (12V going high again should "abort" the 200 second countdown.)

The 555 in its simplest configuration is non-retriggerable. If you look in the datasheet, you will see example circuits where there's a PNP transistor to make it be retriggerable.

Other one-shots are retriggerable without such shennanigans. For example 74HC123. The HC123 has other nice features like choice of inputs for selectable trigger polarity etc.

Tim.

Reply to
Tim Shoppa

Are these 12V relays or 5V relays?

Reply to
Fred Bloggs

All the relays except RY1 Are powered from the 12 V supply. When the supply drops, they drop. When the 12V supply comes on, they energize. That leaves RY1, which you want to drop out 200 seconds after the 12V supply drops out. It is a

5V relay. Here's a circuit that may work for you. It will drop out RY1 after an adjustable delay. +12 ---[R1]---+----+ +---- +5 | | | | | [RY1] | | | [ZD] | c/ | +-----+---| TIP 120 | | | e\\ | [VR1] [C1] | | | | | Gnd (12V) ----+----+-----+-----+--- Gnd (5V)

Zd - 5V 1 watt R1 - 330 1 watt C1 - .1 farad 5.5 v super cap VR1 - 50 K RY1 - 5 volt relay

When the 12 volt supply drops, C1 begins discharging through VR1 and the transistor. When the voltage across C1 falls to about 1.2 volts, the transistor stops conducting, and the relay de-energizes. If the 12 volts supply turns back on before then, the transistor is held on by the supply, and C1 re-charges.

Ed

Reply to
ehsjr

Wow - what a nice, simple, elegant solution. No IC's...........

Andrew VK3BFA

Reply to
Andrew VK3BFA

Ooops - forgot the "backwards" diode (1N400x) across RY1

+12 ---[R1]---+----+ +-----+---- +5 | | k| | | | [D1] [RY1] | | | | | | +-----+ | | | [ZD] | c/ | +-----+---| TIP 120 | | | e\\ | [VR1] [C1] | | | | | Gnd (12V) ----+----+-----+-----+--- Gnd (5V)
Reply to
ehsjr

Thanks! Ed

Reply to
ehsjr

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