I have been looking at schmitt trigger circuits and didn't realise how many variations there are.
Anyway I found this one:
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The author here says that by adding a 22k resistor to the bottom diagram turns into a schmitt trigger circuit as the top diagram.
From other schmitt trigger circuits I have see on youtube and other sites, this one is so simple, but works. The switching from off to on is almoast instant for the 2 leds. But I have a question here, it says this:
Two LEDs are needed because, when the Op Amp pin 6 voltage is low, there may still be as much as two volts present. This could be enough to light a single LED. With two LEDs, four volts are needed. The Op Amp pin 6 voltage will be well below 4V so the LEDs will be off.
WHat afctually happens is both leds come on at the same time and the voltage is 2.18v when on.
I am using a 33k resistor instead of a 22k which I don't have, if that makes the difference.
What are the high and low voltages you get on Pin 6?
Usually they will come on at very close to same time, there will be a slight difference of the voltage across them, but they will be pretty similar. If you are using a common red LED they will have a voltage drop of about
1.8v, so 3.6V for to with the rest dropped across your 560 ohm resistor. The voltage drop is depending on the current through the LED, but the
1.8V for a red LED is close.
You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6 switches, both high and low. Then put another 33k in parallel with the one you have and repeat the voltage records as above. Then you can tell me what difference changing the value of the feedback (your 33k) resistor causes. I like this you are one upping me and removing even more parts. You have been thinking. Mikek
The voltage on pin 6 , as the circuit is now, or per the diagram/schematic, is 2.0v when Lo, and 4.0v when Hi
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.
Another 33k resistor was placed in parallel with the one across pins 3 and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6. On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
other schmitt trigger circuits I have see on youtube and other
switching from off to on
e of your eye!
it says this:
en the Op Amp pin 6 voltage is low,
l be as much as two volts present.
ough to light a single LED. With two LEDs, four volts
are the high and low voltages you get on Pin 6?
the same time and the
same time, there will be a
e voltage across them, but they will be pretty
lar.
e a voltage drop of about
he voltage drop is depending on the current through the L ED, but the
h I don't have, if that
Pin 6 when Pin 6
put another 33k in parallel with the
nd repeat the voltage records as above. Then you can tell
you are one upping me and removing even more parts.
is now, or per the
, and 4.0v when Hi
en Lo, but does fluctuate upto 10.5v
Hi the voltage is 10.6v, and again does fluctuate upto
lel with the one across pins 3
was 2.0v and the Hi voltage was 3.2v on pin 6.
3, the voltage when Lo was 10.2v and when Hi the voltage was
Just want to say some words. Schematic is strange, and de scription is strange too. Author said: "Adding the 22K resistor adds some positive feedback and the on and off
threshold voltages will now differ slightly. Adding this resistor converts the circuit from a Comparator to a Sch mitt Trigger."
That's not true, because threshold volt age will be the same: it will be half of power voltage, value at point A. Feedback does not change threshold vol tage, it just changes voltage/current through LDR, so be haviour of schematic depends on Volt-Ampere characteristi cs of LDR. If characteristic is like classical resistor (resistance does not depend on voltage/current, just lig ht; I have not work with LDR), it will make LDR resistan ce thresholds, which makes lighting thresholds.
voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
again does fluctuate upto
There is somethin g wrong: with schematic on board or OP AMP or while meas uring. Check voltage at point A, directly on pin 2, may b e R2 is not connected properly. Voltage on pin 2 should be ~6V. Schematic should turn turn on/off leds when Volt age on point B is near voltage on point A. You may measu re resistance of LDR when it is "in dark" and when it is "in light". Do not forget to remove it from schematic fo r measuring.
In addition there is a mistake on the sch ematic of board: there is no
Ok, I boobed by putting x2 incorrect resistors. Doh!!! I used x2 47k resistors in error and have changed them to the 10k resistors.
At pin 2 the voltage is 6.1v. The voltage at pin 6 is 10.5v when both leds are on using x2 10k resistors.Putting light onto the ldr turns the leds off and the voltage at pin 6 is 1.98v Where it should be x2 10k ones I used a 10k potentiometer, and gives the same results
Those are just a voltage divider, setting the voltage on Pin 2. I suspect you could you could use a resistor value up to 500K and have little change in the voltage on Pin 2. I found the input resistance of a 741 is 2Mohm, so, I did the calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v. It would raise Pin 2 to 5v. (Side no, there may be different style 741s, with different input impedance, I don't know.)
Do you need to know how to shift the output level ( 6.1v to 1.98v) so you can put a buzzer on the output? Mikek
Err, yes I would like to know that. When the 12v is applied the leds are both on, and go off when light is shone on to the ldr, so I'd like the buzzer to sound when the leds go off.
It's not a good idea to set huge resistance on high-impedance imput: AMP can receive radio noise on input. 10K is enough normal resistance, I think. With R1 and R2 47K it should work too, but I do not prefer such resistance: too high in high-impedance input. It looks like that there was a problem with badly connected R2, because when leds are off voltage on pin 3 should be below ~6V (voltage on point A), but he had:
So would I! I might, from what you said below, it seems you need to voltage shift and invert, I'm not sure that can be done with just one transistor. ( but you know with what you have you could trigger a 555 and have clean 0 to 12v swing. (I haven't thought if it needs an inversion.) But the 555 does do your level shift.
Let me restate that as a question. Do you want the the buzzer to buzz when light shines on the ldr? Your previous circuit was opposite.
Oh, you are now using 12V? That shifts the numbers for that voltage divider up to 6v and without redoing my calculation, maybe to 6.7 with the 500k. Mikek
All that is possible, 500k is not huge, It would actually be 250k to ground, but, I was trying to teach him that, he is just setting the voltage on pin 2 with those resistors. The ratio is important not the absolute value.
It looks like that there
I think he is just letting light leak into his ldr giving him that fluctuation. Mikek
I was only using a 12v supply because that was what was shown on the circuit on the website. Actually, I have now used a 9v battery supply and the readings are now different.
On Pin 2 the voltage is 6.63v On Pin 6 the voltage is 7.36v. As before, putting light onto the ldr to make the leds go ff, the voltage is 1.9v
Yes I would want the buzzer to buzz when light shines onto the ldr.
If you are using two 10k resistors, that should split your supply voltage in half, or 4.5V. Are you sure it's Pin 2 and are you sure you have 2- 10k resistors. Also what it the exact part number and manufacturer of the 741.
Don't know why you got more output range with lower Vc.
I would just use the 555, but If if I took the time I think I could make it work with two transistors. But I would need to experiment, not design. Mikek
Mmm, strange that, but yes when the light shines on the ldr. I have the led lighting up when light shines on the ldr, but I am not sure where to connect the buzzer to. Connecting it to pin 3 on the 555 it buzzes with a low pitch and the led lights dimly, then when I shine light on the ldr the buzzer really buzzes and the led is in full brightness.
Yes if it is possible. This circuit using the 741 op amp has the led shining when light hits the ldr, and this is the one where iI don't know which pins or where to connect the buzzer. If it can't be done, then it can't be done, and if it needs a 555 to make the buzzer work, then I may as well just use the schmitt trigger circuit you sent me.
When the ldr is on the positive rail and the is is in the negative rail, the voltage on pin 6 is as follows: when the ldr is unshaded the voltage is 1.93v when the ldr is shaded the voltage is 2.5v
When the ldr is in the negative rail and led in the positive rail The voltage on pin 6 is as follows: When the ldr is unshaded the volatge is 4.97v When the ldr has light on it, the voltage is 7.46v
Are you sure you used the same resistor in both situations?
You changed your series resistor from your first circuit to second circuit from 100k to 2.2k. That's a big change. You/I/we need a little better understanding of the ldr you have. In other words, what is the resistance in complete dark and what is the resistance with the amount of light you expect to hit it in a normal situation. I think the can do that with your DVM. Measure it in both light and dark. Give me those numbers.
Now set up a circuit Vc--33k resistor--ldr--ground. Now give me VC, and the the ldr/resistor voltage in the dark. and Now give me VC, and the the ldr/resistor voltage in the expected light.
I expect after doing the calculations I will find they are the same resistances. Just making sure there is no current related characteristic.
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