The cap in the base circuit needs a discharge path. A resistor to GND from either end of the base resistor will do the trick.

Actually, the only place that a discharge resistor (in that circuit) can work is on the input to ground.

 

Actually, the only place that a discharge resistor (in that circuit) can work is on the input to ground.

Nope. The base-emitter junction is a rectifier, allowing current to flow in only one direction.
If the base circuit RC time constant is much less than the pulse width, the cap will charge up on the first pulse, and will not discharge. You will get one output pulse on the collector, and then the transistor will remain off.
If the time constant is long compared to the pulse width, then a few pulses will be require to charge the cap, and then the transistor will remain off.

 

Nope. The base-emitter junction is a rectifier, allowing current to flow in only one direction.
If the base circuit RC time constant is much less than the pulse width, the cap will charge up on the first pulse, and will not discharge. You will get one output pulse on the collector, and then the transistor will remain off.
If the time constant is long compared to the pulse width, then a few pulses will be require to charge the cap, and then the transistor will remain off.

While an interesting argument, all of that has nothing to do with why that cap will not discharge. Imagine for a moment that you connected a battery from GND to the cap. The cap charges through the base resistor and the base emitter junction to ground. The cap is now charged. Now remove the battery. Why would that cap discharge? There's no path!

 

While an interesting argument, all of that has nothing to do with why that cap will not discharge. Imagine for a moment that you connected a battery from GND to the cap. The cap charges through the base resistor and the base emitter junction to ground. The cap is now charged. Now remove the battery. Why would that cap discharge? There's no path!

A source won't necessarily go to an open circuit in one state (as I'm sure you know), but it certainly can, such as with a switch, or an open collector PNP. In that case, you are correct about needing a pulldown on the input. That still won't solve the problem. You still need a resistor to ground on the other side of the cap.

 

A source won't necessarily go to an open circuit in one state (as I'm sure you know), but it certainly can, such as with a switch, or an open collector PNP. In that case, you are correct about needing a pulldown on the input. That still won't solve the problem. You still need a resistor to ground on the other side of the cap.

It's always good practice to not leave the base floating.. and yes the base emitter junction would prevent discharge without it. I would not use this method anyway. The trigger circuit that I posted is the most common method to trigger the 555.

 

The trigger circuit that I posted is the most common method to trigger the 555.

I agree, except it does not meet the accepted definition for a retriggerable monostable. I don't mean to imply that Wikipedia is an unimpeachable source.

 

I agree, except it does not meet the accepted definition for a retriggerable monostable. I don't mean to imply that Wikipedia is an unimpeachable source.

If you're referring to retriggerable, in the sense of resetting the output pulse to its quiescent state prior to it completing its cycle, then no. I don't think the OP needs that though. If he did need it, it's is easy to do with the 555 by integrating the Trigger pin with the Reset pin.

 

If you're referring to retriggerable, in the sense of resetting the output pulse to its quiescent state prior to it completing its cycle, then no. I don't think the OP needs that though. If he did need it, it's is easy to do with the 555 by integrating the Trigger pin with the Reset pin.

True, but in this case, I don't think implementing that method would be quite as simple as the circuit Bill posted. The trigger and the reset both have to be negative, and the reset has to be shorter than than the trigger. This would require an inverter and two differentiators - unless you can think of a simpler method.

 

The cap in the base circuit needs a discharge path. A resistor to GND from either end of the base resistor will do the trick.

True enough. I suspect the drawing you showed would work, since the OP may not be aware that retriggerable has a specific meaning in the world of monostables. However, since there is a chance he does mean it...

Wish I could remember the schematic for the missing pulse detector.

At this point we're waiting for the OP to get back with us.
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Bill, a retriggerable oneshot is a missing pulse detector if you make the pulse width greater than the period of the input, but less than two periods.

 

This I know. Thing I don't like about what I drew is it is fundimentally an analog input. I'm thinking that Forrest had a better way. Usually I can hang on to schematics in my head, but this one got away.

 

I don't understand what you mean by "analog input".

 

It is possible not to drive the transistor to saturation, which will not discharge the capacitor fully. Basically we're looking at RTL logic here.

 

True, but in this case, I don't think implementing that method would be quite as simple as the circuit Bill posted. The trigger and the reset both have to be negative, and the reset has to be shorter than than the trigger. This would require an inverter and two differentiators - unless you can think of a simpler method.

I'm not sure which of Bill's circuits you're referring to. What post# was it in?

Here's a method that employs the reset pin to reset the 555 to its initial state prior to completing its output cycle. Therefore the next pulse will retrigger it. Two plots are shown. The left graph shows the 555 completing it's RC timing cycle because VG1 produces only one trigger pulse. The right graph shows the 555 not completing it's RC timing cycle because VG2 sent a reset pulse.

Side note: It might be argued that this does not satisfy the definition of retriggerable. Are we referring to the following criteria?

A monostable (Pulse Stretcher) who's output state changes upon arrival of an input pulse but will never complete an output cycle as long as another pulse arrives at the input prior to the the expiration of the RC time constant? I think this loosely fits the definition of a Missing Pulse Detector.

 

I'm not sure which of Bill's circuits you're referring to. What post# was it in?

I was referring to post 29, which I believe is the only circuit he posted in this thread, except for the original version of the same circuit, which did not have the resistor from base to GND.

Here's a method that employs the reset pin to reset the 555 to its initial state prior to completing its output cycle. Therefore the next pulse will retrigger it. Two plots are shown. The left graph shows the 555 completing it's RC timing cycle because VG1 produces only one trigger pulse. The right graph shows the 555 not completing it's RC timing cycle because VG2 sent a reset pulse.

Side note: It might be argued that this does not satisfy the definition of retriggerable. Are we referring to the following criteria?

A monostable (Pulse Stretcher) who's output state changes upon arrival of an input pulse but will never complete an output cycle as long as another pulse arrives at the input prior to the the expiration of the RC time constant? I think this loosely fits the definition of a Missing Pulse Detector.

A retriggerable monostable restarts the timing cycle every time a trigger pulse is applied. Below is another way of doing that. Bill's circuit is slightly simpler.

 

It is possible not to drive the transistor to saturation, which will not discharge the capacitor fully. Basically we're looking at RTL logic here.

I don't see the problem. All you need to do is design the circuit so the transistor does saturate.

 

Not knowing the OP signal source makes that a bit of a challange, don't you think? I think we scared him off...

 

I was referring to post 29, ....

That's too funny. When Bill posted that schematic the conversation immediately deviated to the base circuit issue so I completely missed the 555 retriggerable hookup!

 

By the way, the missing pulse detector just uses a PNP with it's emitter connected to the timing cap, collector to GND and base to trigger pin.

 

By the way, the missing pulse detector just uses a PNP with it's emitter connected to the timing cap, collector to GND and base to trigger pin.

The guy that designed this circuit forgot to consider base-emitter breakdown. For Vcc>≈1.5*Vbreakdown, the timing will be altered by the breakdown.
If the trigger is capable of providing a lot of current in the high state, and the high state voltage > 2/3 Vcc, the transistor and/or the 555 and/or the source may be damaged. My bet is on the transistor.