transistor timer circuit

[THEONE]

I found this one timer circuit, but it uses PNP transistors

I want to ask if it is possible to modify it with NPN transistors and if it is actually work

 

P.S. at that page also it says how it works but still i can nt understand it, can somebody explain to me little bit better...

[WSM]

I found this one timer circuit, but it uses PNP transistors

I want to ask if it is possible to modify it with NPN transistors and if it is actually work

P.S. at that page also it says how it works but still i can nt understand it, can somebody explain to me little bit better...

 

I would try it the way it's designed before modifying it.

 

The basic way it works is as an RC timer (resistor/capacitor). Depending on the value of the resistor and capacitor, it takes so much time to fully charge the plates of the capacitor and this function allows us to create timers. Inversely, a bleeder resistor across the leads of a capacitor takes so much time to fully discharge the capacitor when the current stops flowing.

 

A study of DC theory would help you gain an understanding of the fundamentals (and practicing on simple circuits would cement that understanding). An excellent source for learning is: Basic Electricity by Barnes & Noble (ISBN 0-7607-5238-9) and is a reprint of the Bureau of Naval Personnel Training Manual.

 

Good luck.

 

WSM

[THEONE]

I have mad this one with NPN transistors, is it correct ?

Also i am planning to add a N Channel MOSFET in case of SCR

Edited July 25, 2012 by THEONE

[Peret]

I have mad this one with NPN transistors, is it correct ?

Also i am planning to add a N Channel MOSFET in case of SCR

No, that circuit won't work at all. The original circuit works because the capacitor can charge through the base-emitter junction of Q1. As long as the charging current flows, Q1 is turned on and is clamping Q2's base to emitter thus keeping it turned off. When the capacitor is fully charged, current no longer flows out of Q1's base so Q1 turns off, allowing Q2's base to be pulled low by R2, thus turning on Q2 to fire the SCR. The circuit is entirely dependent on PNP transistors. Substituting NPNs as in your second circuit has reversed the emitters and collectors, so now there's no charging path for the capacitor and even if there was ,in that configuration they don't work the same as the PNPs in the original. If you want to use NPNs you have to keep the emitters and collectors in the same place and reverse the polarity of the battery and capacitor, but then the SCR won't work.

 

Note, you cannot substitute a MOSFET. It won't turn on properly at 3V.

[THEONE]

No, that circuit won't work at all. The original circuit works because the capacitor can charge through the base-emitter junction of Q1. As long as the charging current flows, Q1 is turned on and is clamping Q2's base to emitter thus keeping it turned off. When the capacitor is fully charged, current no longer flows out of Q1's base so Q1 turns off, allowing Q2's base to be pulled low by R2, thus turning on Q2 to fire the SCR. The circuit is entirely dependent on PNP transistors. Substituting NPNs as in your second circuit has reversed the emitters and collectors, so now there's no charging path for the capacitor and even if there was ,in that configuration they don't work the same as the PNPs in the original. If you want to use NPNs you have to keep the emitters and collectors in the same place and reverse the polarity of the battery and capacitor, but then the SCR won't work.

 

Note, you cannot substitute a MOSFET. It won't turn on properly at 3V.

 

Can you make to me this diagram with NPN transistors ?

I am going to use a 9V battery

[Arthur]

As your basic component level knowledge is lacking, I suggest that you look into using a 555 timer chip There are lots of circuits on the web a reasonable R and C will get you the delay you want Lots of effort is needed to keep the weight down.

[THEONE]

I have already made 2 times based on 555... And even thew the electronics and the circuit was exactly the same, the results was different. I do not know why

This circuit It looks simpler cheaper and easier to play than 555 timer based, that is why i want it

Edited July 26, 2012 by THEONE

[dagabu]

Really? Even I can get them in bulk for $.15 each, SMD or through hole. And Peret knows how electronically retarded I am too.

 

-dag

[Peret]

It's against my better judgement, coz it took me more effort to draw this than it would have taken you to get a couple of PNP transistors, but try this.

[THEONE]

It's against my better judgement, coz it took me more effort to draw this than it would have taken you to get a couple of PNP transistors, but try this.

 

Thanks alot Peret, i will try it today and i will post the results.

 

P.S. I am going to use a N Channel MOSFET in case of SCR, is that ok ?

[THEONE]

Here is the circuit you give to me but it doesn't work for me

I am posting a picture for details

[Chuleo]

Just a quick question: Are you planning on using multiple of these devices?

And a quick advice: Consider using a breadboard or a veroboard for circuit prototyping as it makes the whole proces much cleaner.

[THEONE]

Just a quick question: Are you planning on using multiple of these devices?

And a quick advice: Consider using a breadboard or a veroboard for circuit prototyping as it makes the whole proces much cleaner.

 

At this point no, but in the future who knows...

i try it like this first to see if it works and after i use breadboard

[mike_au]

i try it like this first to see if it works and after i use breadboard

 

Are you confusing breadboard with a PCB or circuit board? Using a breadboard to build your prototype would be much easier than what you are doing.

[THEONE]

Are you confusing breadboard with a PCB or circuit board? Using a breadboard to build your prototype would be much easier than what you are doing.

 

Yes you have right

Unfortunately i do not have breadboar

[mikewj]

No, that circuit won't work at all. The original circuit works because the capacitor can charge through the base-emitter junction of Q1. As long as the charging current flows, Q1 is turned on and is clamping Q2's base to emitter thus keeping it turned off. When the capacitor is fully charged, current no longer flows out of Q1's base so Q1 turns off, allowing Q2's base to be pulled low by R2, thus turning on Q2 to fire the SCR. The circuit is entirely dependent on PNP transistors. Substituting NPNs as in your second circuit has reversed the emitters and collectors, so now there's no charging path for the capacitor and even if there was ,in that configuration they don't work the same as the PNPs in the original. If you want to use NPNs you have to keep the emitters and collectors in the same place and reverse the polarity of the battery and capacitor, but then the SCR won't work.

 

Note, you cannot substitute a MOSFET. It won't turn on properly at 3V.

 

 

I was just browsing for relevant posts for my project and I happened to stumble upon yours. thanks for your useful input. It's appreciated.

 

 

mike

[THEONE]

On the first circuit, with the PNP transistors, what i should change to add a 9 volt battery in case of 3 volt ??

[BengalFlair]

On the first circuit, with the PNP transistors, what i should change to add a 9 volt battery in case of 3 volt ??

 

I made some modifications to the original circuit and tested it with a 9 volt battery. It works great! I added two resistors each of 1k ohm to limit the base current of Q1 and collector current of Q2 as well as gate current of the SCR for protection of those devices. Here is the modified circuit with components which I used.

Edited December 1, 2012 by BengalFlair

[THEONE]

Thanks a lot bengalflair, i will try it and i will tell you the results.

Generally it looks more promising than the 555 timer circuit

[THEONE]

What if a MOSFET will be used in case of SCR ?

[THEONE]

Ok i have just finish the circuit but it doesnt work for me.... :(

 

I remade the circuit for better construction but it doesnt work... can anybody see any mistake here ??

Edited December 3, 2012 by THEONE

[BengalFlair]

I replaced the SCR with a n-channel MOSFET (IRF640N) which was in hand and again added a 10k ohms resistor. I used a miniature Christmas lamp as load. The circuit is also working fine as previous.

It is very important to identify the proper terminals (C, B, E and G, D, S, etc) of the devices and I think there might be a mistake as your diagram especially with the transistors.. If you use LED as load for testing then check for correct polarity. I have made a diagram showing the proper position of the terminals and I think it will help you.

 

Edited December 4, 2012 by BengalFlair

[THEONE]

I saw this picture and i worked based on this picture... May be it will be wrong... My transistors are 2 c558b ph80 and the mosfet is the IRFZ44N

[BengalFlair]

There are various pin configurations and packaging of transistors but bc558b or c558b has the same as I have shown in my diagram. Here are some PinOut of transistors.

[THEONE]

I changed the collector - emitter and it finally works!!! Thanks a lot ;) !!