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Pulse Stretching

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saiello

New Member
Hi All,
I need to stretch out a 5V roughly 1uS pulse from a photodiode to about 200uS so that subsequent circuitry can detect it. The shape of the stretched pulse doesn't matter as long as the 'stretching' circuitry can respond quickly to the photodiode input and then let the voltage decay over a 200uS time interval. One way I thought of doing this was to have the photodiode as input to a diode, the output of which goes to a capacitor and resistor in series. The capacitor is grounded and the 200uS output pulse is read from the output of the resistor. Would this work?

Thanks.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi All,
I need to stretch out a 5V roughly 1uS pulse from a photodiode to about 200uS so that subsequent circuitry can detect it. The shape of the stretched pulse doesn't matter as long as the 'stretching' circuitry can respond quickly to the photodiode input and then let the voltage decay over a 200uS time interval. One way I thought of doing this was to have the photodiode as input to a diode, the output of which goes to a capacitor and resistor in series. The capacitor is grounded and the 200uS output pulse is read from the output of the resistor. Would this work?

Thanks.
hi,
Using a cap to stretch a pulse in this way would not give the result you would expect.

A 555 ic could be configured to give a 200uS pulse from a PD input.
 
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colin55

Well-Known Member
You can use a transistor to lengthen the pulse. The diagram you provided above would not work. You need a transistor to have a "strength" to charge the capacitor quickly.
 
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Sceadwian

Banned
Would you care to post a schematic colin?
 

colin55

Well-Known Member
Personally I would use a 20 pin microcontroller, running at 20MHz and a 5amp power supply with 20AH battery back-up.

You have obviously NEVER used a 555. Try to get 5v out of a 555.
 
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Sceadwian

Banned
Odd colin, I've never seen you mention micro controller circuits before.
 

Sceadwian

Banned
Try to get 5v out of a 555? What's that mean? Many 555's are good up to 15 volts...
 

Sceadwian

Banned
A cmos 555 should go rail to rail, though not with as much output current or total voltage allowance.
 

Speakerguy

Active Member
You have obviously NEVER used a 555. Try to get 5v out of a 555.
LOL! Funniest thing I have ever read on this forum. I have a BS in Electrical Engineering from a top 10 engineering school in the US (with a GPA of 3.7/4.0 to boot). I've been working in electronics design for about four years now.

Please read the following page for the extremely common LMC555 CMOS timer and note the line "Output fully compatible with TTL and CMOS logic at 5V supply".

LMC555 - CMOS Timer

I think even Radio Shack carries these.
 
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colin55

Well-Known Member
If you knew what you were talking about, why suggest a 555 to someone who doesn't know the difference between a 555 and 7555? You are supposed to be helping someone who has a problem.
 

Sceadwian

Banned
Does 7555 include 555 as a part number? Okay now we're just splitting hairs. The 7 prefix is widely used for CMOS parts.
 

Sceadwian

Banned
By the way colin there is a VERY good chance the drop from a standard 555 will be allowable for the users logic levels.
 

colin55

Well-Known Member
Try and get a reliable "HIGH" reading at 3.5v with TTL.

If you think a 555 is equal to a 7555, then your university training will say you can use a 7414 in place of a 74C14.
 
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