# 741 Timer

Discussion in 'Homework Help' started by David Waddell, Dec 11, 2014.

Dec 3, 2014
24
1
This is a question in a Task. I am abit confused on the answer and would like help.

I built the circuit on breadboard (Circuit diagram in attachments) I have it set up so when no light is entering the LDR, the LED turns on.

The question I am being asked -

(i) Confirm the operation of the circuit and discuss how the circuit operates. Use a Multimeter to measure the voltages at pins 2, 3 and 6 of the 741 op-Amp.

I used a mutlimeter and got the following results. (In attachments)

This is my answer so far but I dont know what to write for Pin 6.
In Pin 3 the refernece voltage stays the same in light and dark at 2.54v.

Pin 2 has an input of 1.8v when the LDR has light entering it. This is because the LED is not on and less power is needed. When no light is entering the LDR, the input into pin 2 increases to 4.0v. this is because the LED is now on and needs more power.
When light is entering the LDR, Pin 6 has an output of 4.5

Am I right with Pin 3 and Pin 2? And can you help with Pin 6?

Thanks

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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,234
565
hi,
Pin 3 as you say is the reference voltage and should remain constant.
Pin2 voltage is either Higher or Lower than the pin 3 ref voltage, depending if the LDR has light shining on it.

The 741 is wired as a comparator, which means if pin2 voltage is lower than pin3 voltage, pin 6, the output, will be high, approx 4.5V
When pin2 voltage is higher than pin3, pin 6 will be Low approx 1V

Repost your modified answers so we can see if you understood OK.

E

Dec 3, 2014
24
1
I have attached my multimeter results, I still dont understand Pin 6?

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,234
565
hi,
A 741 operational amplifier is an old design and has a very poor specification.
For example, a High output pin #6 , will always be about 1.5V less than the supply voltage, also pin #6 will never go below about 1V when the 741 is powered from a single supply. So the output range for a 5V supply is ~3.5V and ~1V.

As I explained, the 741 connected in that way, ie: as a comparator, compares the voltages on the input pins #2 and #3 and sets the pin #6 output high or low depending upon the result of the comparison.
ie: if pin 2 is higher than pin 3, then pin#6 is +1V... if pin 3 is greater than pin 2, then pin #6 is 3.5V.

Is this what you are asking.?
E

5. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,661
1,528
I think you need to measure your LDR. You will find it is NOT as you have simulated it.

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,070
Will the crummy old '741 even work at all with a supply voltage of 5V?

Nope, I just looked it up. It is not expected to work with supply voltage less than 10V. At that, the output swing is expected to be from ~7V to about 3V...

You really need to ditch the '741 and use a modern cmos rail-to-rail opamp. Or, use a supply voltage on the 741 of 20 to 30V.

Last edited: Dec 11, 2014