# IR Receiver / Comparator Circuit help

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#### FMICW

##### New Member
First things first, this is a college assignment. I appreciate that people are not going to do my homework for me, but i am just asking for some guidance/research advice.

I have a pulse width modulation circuit that uses 2 555's with the output of the monostable 555 being an LED. The receiver/comparator is as follows-

The areas i must explain are-
- The resistor value RD before the LED.
- The F.E.T function.
- The PT function.
- The 3K9 resistor.

I will probably be ok with the PT and FET explanations, but struggle more with the reasoning of resistor values.
Can anybody recommend some online resources where i can learn the basic functions covered above?

Thanks

#### ericgibbs

##### Well-Known Member
First things first, this is a college assignment. I appreciate that people are not going to do my homework for me, but i am just asking for some guidance/research advice.

I have a pulse width modulation circuit that uses 2 555's with the output of the monostable 555 being an LED. The receiver/comparator is as follows-

The areas i must explain are-
- The resistor value RD before the LED.
- The F.E.T function.
- The PT function.
- The 3K9 resistor.

I will probably be ok with the PT and FET explanations, but struggle more with the reasoning of resistor values.
Can anybody recommend some online resources where i can learn the basic functions covered above?

Thanks
hi,
Its difficult to pick out a general tutorial for these values.

The resistor values are usually determined by checking the datasheet for the active devices.
ie: the PT data will determine the collector value, to suit the required sensitivity, saturation and speed of operation.
Likewise, one the output signals from PT are known, then the Vref for the comparator thresholds can be determined.

for example: the 5V6 zener feeds a resistive divider of 100K and a 10K variable. The junction of these two will be at +0.509V, this means the voltage level output from the PT when 'lit' will have to fall to somewhere less than +0.5V in order for the comp output to change state.. the actual value is et by the 10K.

Can you say what you think the function of the 2N3819 is.?

#### FMICW

##### New Member
To be honest, your final paragraph explained the comparator logic quite well. The part i can't figure out is what effect the F.E.T has on this comparator input.

I beleive fet's give a high current output for very low voltage/current input... so i can't really see how it works when it is wrapped around itself as in the diagram.

#### ericgibbs

##### Well-Known Member
To be honest, your final paragraph explained the comparator logic quite well. The part i can't figure out is what effect the F.E.T has on this comparator input.

I beleive fet's give a high current output for very low voltage/current input... so i can't really see how it works when it is wrapped around itself as in the diagram.
hi,
Its connected as a constant current source, to supply the 5V6 zener.
The 2N3819 isnt cheap, IMO a suitable current limiting resistor would the job.

The 3K9 in the comp output is a 'typical' value, the LM339 will sink about 3mA with that as a load.

The common range of LED's require a current limiting resistor, currents in the order of 10mA thru 20mA are typical.

The led emitter in your cct with a 100R, suggests a pulse current in the order of ~ 100mA

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#### audioguru

##### Well-Known Member
The gate of the FET should be shorted to its source then the 1M resistor is not needed.

The current will be the IDss rating of the FET (2mA to 20mA).

#### FMICW

##### New Member
The 3K9 in the comp output is a 'typical' value, the LM339 will sink about 3mA with that as a load.
So, what does the resistor actually do? Surely with the resistor in place, current will always flow from + to the output?

The led emitter in your cct with a 100R, suggests a pulse current in the order of ~ 100mA
thanks for that, what calculation did you use there?

Cheers

#### ericgibbs

##### Well-Known Member
So, what does the resistor actually do? Surely with the resistor in place, current will always flow from + to the output?

thanks for that, what calculation did you use there?

Cheers
Even with gate connected to the source the FET will be conducting, it a Depletion mode FET, that means the gate voltage is used to turn the FET off.

Your emitter data sheet gives a 2V fwd drop for the diode, assuming a 12Vdc supply -2V =10v across a 100R resistor gives 100mA.

#### FMICW

##### New Member
Thanks for that, where did you look for the datasheet?

#### Ubergeek63

##### Well-Known Member
Even with gate connected to the source the FET will be conducting, it a Depletion mode FET, that means the gate voltage is used to turn the FET off.

Your emitter data sheet gives a 2V fwd drop for the diode, assuming a 12Vdc supply -2V =10v across a 100R resistor gives 100mA.
Eric,

You need to remember that a lot of these young whipper-snappers have never heard of depletion mode. ... so they would have no clue that it is a current source.

Dan

#### ericgibbs

##### Well-Known Member
Eric,

You need to remember that a lot of these young whipper-snappers have never heard of depletion mode. ... so they would have no clue that it is a current source.

Dan
hi Dan.
In his opener the OP said he would be OK with the FET/PT.

Hows the new hotrod... are you now a boy racer.?

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#### audioguru

##### Well-Known Member
So, what does the resistor actually do? Surely with the resistor in place, current will always flow from + to the output?
The LM339 is a comparator with an open collector output. It needs a pullup resistor at its ouput for the output to be high.
Its typical saturation voltage is 0.25V (0.4V max) with a 4mA load so the 3.9k resistor will have a current of (12V - 0.25V)/3.9k= 3mA.

#### Ubergeek63

##### Well-Known Member
hi Dan.
In his opener the OP said he would be OK with the FET/PT.

Hows the new hotrod... are you now a boy racer.?
True but the repeated questioning about it betrayed the truth of the matter. I do not even think about it unless I have an app for it.

Doing great, 98 Saturn PIF saves me about \$280/mo. While I would not call myself a racer I must confess to having missed my 5 speed stick!

Dan

#### hansdals

##### New Member
I see the schematics write "FET" but 2n3819 is actually a JFET, a high frequency amplifier.

You are interfacing digital with analog. Digital is noisy and that interferes with the analog reference (zener 5.6V). To reject the noise from power supply, you supply the zener with a constant sorce. Thats what the 2n3819 is for. If this is really nesessary is up to the application and where the circuit is used.
The JFETs are new to me, but i just now read this tutorial: http://www.electro-tech-online.com/custompdfs/2009/03/jfet101.pdf and i get it that when Vg=Vs, Id=Idss, a very stable current over large range of delta-Vds. Read the tutorial.

#### FMICW

##### New Member
Thanks for everybodies help.

Regarding the value of Rd, what is it's significance on the transistor loading of the BFY51? I am led to beleive that the resistors value effects this as well as the LED current limiting.

Cheers

#### audioguru

##### Well-Known Member
Regarding the value of Rd, what is it's significance on the transistor loading of the BFY51? I am led to beleive that the resistors value effects this as well as the LED current limiting.
If the BFY51 has enough base current to saturate then Rd (and its supply voltage) is the only part that determines the amount of LED current.

#### FMICW

##### New Member
I certainly can't figure it out, when i clarified with my tutor that the resistor limited current to the LED, he said it was more to do with transistor loading. I thought it must be some sort of minimum or maximum current for the BFY51.
No worries, i will sort it!

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