I have an application where I need to send just a pulse output from a varying (sometimes semi-permanent) HIGH signal (from a reed switch in a water meter).

I therefore think that the best way to achieve this is with a rising edge trigger circuit using I think a monostable multivibrator.

From various musings around the internet, including this site, it has all the features that I need and I can set at least an approximate pulse length with a suitable resistor and capacitor.

My main question is what mv to buy, especially given that I would first of all make this up on a strip board. Secondly, is the wiring as easy as it looks?

I have a 12V battery supply that is part of a keyfob RF transmitter and one of the reasons for wanting a short pulse ouput is to preserve battery life.

Please excuse any ignorance above - I am only a lowly Chemical Engineer...

Hope someone can help...

 

The attached circuit triggers the 555 monostable on a zero-to-positive voltage on the input to the transistor. The output pulse width is set by R4-R5-C2 component values.

ken

 

Hi Ken,

Thanks for a quick reply! I think (as a chemical engineer) that I can understand most of the circuit. May I ask just a couple of questions?

Are the capacitor values given in uF?
I'm not sure exactly what C2 is - I think it's an electrolytic and therefore polarised capacitor and the curve on the symbol makes no difference?
I presume that R5 is a variable (pot?) resistor?
The signal comes in through R1?

Also, I don't know whether it's relevant, but ideally the circuit would be non-retriggerable so that if I got a continuous output from the water meter, the 555 would not continue to pulse out.

Thirdly (cheeky I know), I presume that I can feed the whole lot (through R1 and +12V connections) 12V only when the switch is closed, again to avoid a power drain?

Simon

 

So far you are mostly describing a 555. Check the link out.

555 Monostable

 

Hi Ken,

Thanks for a quick reply! I think (as a chemical engineer) that I can understand most of the circuit. May I ask just a couple of questions?

Are the capacitor values given in uF?

Yes.

I'm not sure exactly what C2 is - I think it's an electrolytic and therefore polarised capacitor and the curve on the symbol makes no difference?

Yes. The + sign is an indication if the polarity. The curve is the negative on polarized capacitors. On the old foil wrapped capacitors it was the outside foul.

I presume that R5 is a variable (pot?) resistor?

Yes. The R5 resistor limits the range of the "pot". If you were going for a nonadjustable pulse width, you could eliminate R4 and select the exact value you need for R5.

The signal comes in through R1?

Yes.

Also, I don't know whether it's relevant, but ideally the circuit would be non-retriggerable so that if I got a continuous output from the water meter, the 555 would not continue to pulse out.

The coupling capacitor C1 provides a brief negative-going pulse to the trigger input of the 555 when you switches closed, starting the monostable period. It also couples a brief positive-going pulse when your switch opens, but this is ignored by the 555.

Thirdly (cheeky I know), I presume that I can feed the whole lot (through R1 and +12V connections) 12V only when the switch is closed, again to avoid a power drain?

Simon

Yes. the amount of current during that period is limited by R1. I(mA)=12v/10,000Ω But in that circuit, the 555 is powered buy 12v all the time.

If power usage needs to be limited, you could use a power-up reset monostable circuit, attached. It would only be powered when your switch is closed. If your switch stayed closed for a while, it would be powered during that time, but it would draw no power when the switch is open. There may be a more elegant way to draw no current except for the monostable period...but, I'll have to ruminate on that.

Ken

 

Ken, Bill,

Thanks for all your help on this one. I think I have identified all the parts that I need and will go about building this over the next few days. Having mooched around a bit more, I have found, in addition to the LM555C, a pre-built Monostable Multivibrator IC (Texas Instruments CD4047BMG4), which looks like I only need to add the RC setup to for timing. I think I will try both, as the suggestions here will almost certainly work and I'm only guessing at the IC!

Simon