Author Topic: How to FET  (Read 1546 times)

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Offline bumba000

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How to FET
« on: October 12, 2016, 05:57:07 am »
Hi All,
         I've been working on this for a couple of days now and am ready for help. http://danyk.cz/polic_en.html

I have this working as is. I also have this working by replacing 1M R's with variable resistors to create many different variations of on off times.

This circuit has me a bit confused and I can not find info to help me out because I don't know what what I'm looking for is called. Connecting my meter to pin 3 and pin 3 I get voltage switching from -2.5v to 3.65v and it of course keeps switching. So you may already know what's going on here but here are my findings.

Measuring from Pin3 to ...
Chip A Pin3 to VCC switches from -3.43v to -3.54v
Chip A Pin3 to GND switches from  3.62v to  3.7v
Chip B Pin3 to VCC switches from  -1v     to -7.10v
Chip B Pin3 to GND switches from   0v     to  6v

Chip A Pin3 to Chip B Pin3 -2.50v to 3.65v

I would like to use a couple of mosfets in place of the LED's. I would like to switch a larger load. I have figured out many many ways not to do this, but I have yet to find a way to make it work. I've also tried only N-channel's.

Please help.
Thank you,
John

P.S. I'm not building a police car. I've a much different application.  :)
« Last Edit: October 12, 2016, 06:15:17 am by bumba000 »
 

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Re: How to FET
« Reply #1 on: October 12, 2016, 07:53:35 am »
You're misunderstanding how this circuit works.  Each of the 555 timers produces a square(ish) wave at their respective pin 3's.  The one on the left operates  slower than the one on the right (because C2 is larger than C3). 

Here's how it works:
1> Consider when the 555 on the left has it's output go high.  When the right side 555 output is high, there is no voltage across the LEDs, so they're all off.  When the right side goes low, current flows through the blue LEDs - lighting them up.

2> When the left side 555 goes low, the LEDs are again off when the right side 555 is low.  But, when the right side goes high, current flows through the red LEDs, lighting them up.

3> Since the right side runs faster than the left, you get about a few flashes of the blue and a few flashes of the red LEDs.

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Offline bumba000

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Re: How to FET
« Reply #2 on: October 12, 2016, 08:31:16 am »
Thank you. I kind of understand that. I realize that each chip's pin 3 will go high then low repeatedly. Making it + or -. Either pin3 to the mosfet gate doesn't work. either the led is always on or never on. I did find one configuration using two zener diodes in series between pin3 (of one chip or the other i don't remember) and the fet gate. This worked. But only for one chip, not the other...
 

Offline rob77

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Re: How to FET
« Reply #3 on: October 12, 2016, 10:19:14 am »
if you want to make the circuit "high power" you can't do it with a mosfet.. you need 2half-bridges and drive each half-bridge with one of the "pin 3" outputs and connect the load between the 2 half bridges.
the half bridge drivers must be with dependent high a low side (one input only, not the drivers with separate high a low side input) for example IR25602 as driver and 2 N-chan mosfets forming the bridge.

and even simpler design would be 2 self oscillating half bridge drivers + mosfets . each driver set for a different frequency and you can omit the 555 timers. for example IR25603 drivers.
 

Offline bumba000

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Re: How to FET
« Reply #4 on: October 12, 2016, 10:58:12 am »
Would an L293D stepper driver work?!
 

Offline rob77

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Re: How to FET
« Reply #5 on: October 12, 2016, 11:10:02 am »
Would an L293D stepper driver work?!

technically you can do it with half of the L293D - connect LEDs and current limiting resistor between 2 half-bridge ouputs of L293 and feed the inputs of those from the 555s.

EDIT:

don't forget the L293 has TTL compatible inputs and absolute max input voltage is 7V only ! so you MUST power the 555 timers from 5V supply.
« Last Edit: October 12, 2016, 11:14:42 am by rob77 »
 

Offline bumba000

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Re: How to FET
« Reply #6 on: October 12, 2016, 11:30:58 am »
Right. I've been reading and comparing datasheets on the model you mentioned and the l293d. I was just about to say that I know I can I'm just working out the how.

I see what you're saying about the 5v too. Yeah I would have missed that or just not realized and would have smoked the little guy with a 9v. So I have some LM7805's here that will get me the 5v needed for the 555 to  l293d Vi and then I'll have 9v for the l293d Vcc1 supply.

Sounds good to me. I know the stepper drivers are limited to 2A per channel and if that happens to not be enough I can add more drivers. I think one will do. Okay, I'm going to play now.

Thank you very much. I may have more questions so don't go too far.  ;D
 

Offline bumba000

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Re: How to FET
« Reply #7 on: October 12, 2016, 12:12:22 pm »
It absolutely works!
Thank you!!
 


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