short pulse ---> long hi signal

[huttojb]

Hi all.

This is my first question on your forum. I'm just wondering how will I generate a high signal for 1.5seconds from a short 50mS hi pulse?????

Jason

 

[dknguyen]

You can use a 555 timer monostable circuit. It's like a single pulse generator. Every time you send a pulse to the input, it generates another pulse that remains high for a certain amount of time. You can Google for the circuit- it's one of the two basic 555 timer circuits.

 

[huttojb]

Sorry, I should of said; I didn't want to use a 555 timer (already thought of this) due to cost, I'm gonna be building 50 - 75 of these so I wanted to keep cost to a minimum. I was just wondering is there any cheap and nasty options??

Jason

 

[Nigel Goodwin]

Sorry, I should of said; I didn't want to use a 555 timer (already thought of this) due to cost, I'm gonna be building 50 - 75 of these so I wanted to keep cost to a minimum. I was just wondering is there any cheap and nasty options??

Build a discrete monostable instead - but it's slightly more complicated, two transistors, five resistors and a capacitor (off the top of my head!).

 

[huttojb]

I'll search on the internet now, could you have a look (if you have the time) and give me a point in the direction

Jason

 

[ljcox]

You can do it with a Hex Schmitt Trigger such as the 40106, 74HC14, etc.

You need 2 resistors, one diode and one capacitor per Schmitt.

 

[ljcox]

Here is one way to do it. It is designed for a 10:1 ratio.

You can change the 100k to give you a different ratio.

Change the capacitor for longer or shorter times.

 

[huttojb]

I'm looking at 74HC14, but could you explain more, it looks like this wouldn't work??

Jason

 

[huttojb]

plus the signal is reversed

 

[ljcox]

I just posted a circuit. It does work, I've used it many times.

For example, I used it in a garage door opener. I wanted to make the light stay on for about 2 minutes.

So the delay is started when the up or down button is pressed and remains on for approx 2 min.

 

[ljcox]

plus the signal is reversed

Use another inverter to invert the output.

 

[huttojb]

Could you tell me where the cct is, Is it very expensive to make, don't for get I'm making 50 of these?

Jason

 

[ljcox]

Could you tell me where the cct is, Is it very expensive to make, don't for get I'm making 50 of these?

Jason

The circuit is 5 posts above.

You could also use the same technique with 556 ICs, but there are only 2 per IC so it is probably more expensive.

Do you have to have a positive trigger and a positive output?

If not, then you obtain 6 circuits per 40106.

Alternatively, as someone wrote, you could use a dual Monostable.

Then you only need one R and one C.

 

[huttojb]

I'm using a PIC to send a pulse and I want this cct I'm trying to get to switch a transistor for approx 1.5 seconds.

I got a positive input and I need a positive output.

I've looked at the dual Monostable on the internet and can't find an example to look and see how it works and re-construct on Multisim??

Jason

 

[ljcox]

Look at the data sheet of the Mono.

 

[ljcox]

I'm using a PIC to send a pulse and I want this cct I'm trying to get to switch a transistor for approx 1.5 seconds.

I got a positive input and I need a positive output.

I've looked at the dual Monostable on the internet and can't find an example to look and see how it works and re-construct on Multisim??

Jason

Why can't the PIC generate a "negative" pulse?

 

[huttojb]

any chance for a part number

 

[huttojb]

Why can't the PIC generate a "negative" pulse?

cos if there is an intermittent contact I'll have a output??? I'm using it if to drive a relay but if the relay is on for > 1.5 seconds it gets very hot.

SAFETY

 

[huttojb]

Can I ask. If I have to buy 50 dual Monostable then I might as well buy 50 555 timers???

Jason. Is there any other way?

 

[ljcox]

Can I ask. If I have to buy 50 dual Monostable then I might as well buy 50 555 timers???

Jason. Is there any other way?

For 50 circuits you only need 25 Monos or 25 of the 556s.

It is difficult to give you advice when I don't know the full story. More questions come to mind than I have answers for. For example

Are ther e50 PICs and 50 delays or one PIC and 50 delays?

Why can't the PIC do the 1.5 sec delay?

Does the PIC have any spare i/o?

Could the relay be driven by a PNP transistor? If so, a "negative" output from the delay circuit would work.