How does this work ?

[ahmedragia21]

Hi,i were doing a line follower robot circuit, and use by accident an invert 74HC04 and it worked great !! however i were following a schematic from lynxmotion Line follower and they used a schmitt trigger and i tried to use a schmitt trigger and it didnt work !! , i wonder how the inverter works with analog signals ? is it used as an amplifier there ?! , also the circuit with the inverter didnt want to work with a 10Kpot instead i used a 22Kpot and it worked very nice !! i really wonder .

https://i16.tinypic.com/4bp91sy.jpg

 

[ikalogic]

LDR have very slow response time comparing it to photodiodes or photo transistors.

Using an op-amp instead of the inverter will be much safer and will give you much better noise imunity.

 

[ikalogic]

the 7404, have a fized shreshold, where it switches from 0 to 1.

depending on the value of the resistance of the LDR the ratio it makes with the Pot. resistance.. it may not swing around the shreshold of the inverter.. thus you may allways be too low or too high. this is why the 22K ohm resistor worked better. it all depends on the value of the LDR.

if you were using an op-amp, this wouldn't have been a problem...

edit: also you need a resistance in series with the LED..

 

[ahmedragia21]

i dont think its like that , but i feel that its use as an amplifier, -_-

 

[philba]

you didn't say which schmitt trigger you used but look at the two thresholds. that's probably the issue. It's possible that the ST never got above the high threshold or below the low. Also, I would expect the '04 to have noisy behavior as it will not switch cleanly. that may not make much of a difference in this case, though.

to use a ST with the LDR, you should adjust the divider so that it triggers high and low for your target environment. Use a dmm to measure the dark and light voltages.

However, I suspect your real problem is the LED "pull down" that will keep the output voltage at the Vf of the LED.

I agree that LDRs are not as good as PTs for this application.

 

[ahmedragia21]

i used 74HC14 as ST , but i didnt get any strange behaviour or glitches with the inverter -_- i dont know why really the circuit works very fine with the inverter :s

 

[ikalogic]

even if it works fine, beleive me this circuit is not professional at all.

as pholba and myself said, you need a resistance in series with the LED, and you have to check the datahseet and see the 2 threshholds.. but again, you do not have any control over the amplification. NO it does not act as an amplifier, it acts as a diffrentiator, whose referece is fixed.

do what you want, this design may be simple but its not reliable. tell me, what are you gonna do if the lightning conditions changed to an extent that cannot be adjusted with the POT ??

 

[philba]

At 4.5V Vcc, the 74hc14 has a high threshold of 2.7V (typical) and low threshold 1.8V (typical). Thus, your LDR/Resistor needs to swing from <1.8 to > 2.7V. though I would shoot for something like 1V and 3.5V.

To recap schmitt triggers, the output goes high (or low for an inverter) when the input goes above the high threshold and does not go low (or high) until the input goes below the low threshold. this is called hysteresis and prevents noise at the switch point from "chattering" the output.

The 74hc04, with a 4.5 Vcc, switches at around 3.15V. You probably never got below the low threshold of 'HC14.

 

[ikalogic]

Doesn't anybody here agrees that's its much pro. to use an op-amp?? and will give more flexibility??

 

[philba]

I don't think pro would be applied to the use of an LDR in this application.

If it were me, I'd use an IR LED and IR phototransistor with a daylight filter in place. With the right pull up resistor, you could directly drive any logic input. No opamp needed.

 

[ahmedragia21]

ya philpa , i agree with ya im going to buy a pair of fairchild reflective sensor IR tx,rx ..

another question , for unpredictable ligh condition , the pot works or there's another better method ?

 

[ahmedragia21]

ya philpa , i agree with ya im going to buy a pair of fairchild reflective sensor IR tx,rx ..

another question , for unpredictable light conditions , the pot works or there's another better method ?

 

[barry995]

OPAMP power supply can I use +15v to 0V?

Hi I have built plenty of circuits in school but I always had a +/- 15V supply. Now that I want to build circuits at home I do not have a supply that goes negative (I do have a 24V DC supply) Can I use it to power an OPAMP that wants +/_15V? by giving it +24 Vcc and 0V Vee or must I build a -12V supply.

Please reply if you have come across this problem before

thanks

 

[audioguru]

Hi I have built plenty of circuits in school but I always had a +/- 15V supply. Now that I want to build circuits at home I do not have a supply that goes negative (I do have a 24V DC supply) Can I use it to power an OPAMP that wants +/_15V? by giving it +24 Vcc and 0V Vee or must I build a -12V supply.

Please reply if you have come across this problem before

thanks

Please don't double-post. I answered this question about opamps in your other post. I also answered the same question recently from somebody else.

 

[ikalogic]

Hi I have built plenty of circuits in school but I always had a +/- 15V supply. Now that I want to build circuits at home I do not have a supply that goes negative (I do have a 24V DC supply) Can I use it to power an OPAMP that wants +/_15V? by giving it +24 Vcc and 0V Vee or must I build a -12V supply.

Please reply if you have come across this problem before

thanks

I'll save audioguru the trouble, (as far as i know... )

You can. you can use an LM358, which does not need negative supply. i suppose you worked on 741 opamp.

Also if you really need 12/-12, you can connect the 24/0 v to the opamp, work as if it was 12/-12, and each time you need a 0V, a simple voltage divider will do.

 

[ikalogic]

ya philpa , i agree with ya im going to buy a pair of fairchild reflective sensor IR tx,rx ..

another question , for unpredictable ligh condition , the pot works or there's another better method ?

yes, the pot works, but you need a pot of high value, say 100K, so that it gives you a wide working spectrum. and also it will be compatible with many types of LDRs.

Ahmed, why you hate op-amps?

 

[audioguru]

you can use an LM358.

This is 2007. An LM358 is too old today:
1) It is noisy. Low noise opamps have been available for many years.
2) It has crossover distortion because it was the 1st "low power" opamp so they left out bias current for its output transistors. Even guitar fuzz people don't like its fuzzy sound.
3) It has a full output bandwidth to only 6kHz like an old AM radio (remember them?). Modern opamps go to 100kHz.

Opamps don't need a negative supply if their inputs are biased correctly.

 

[ikalogic]

This is 2007. An LM358 is too old today:
1) It is noisy. Low noise opamps have been available for many years.
2) It has crossover distortion because it was the 1st "low power" opamp so they left out bias current for its output transistors. Even guitar fuzz people don't like its fuzzy sound.
3) It has a full output bandwidth to only 6kHz like an old AM radio (remember them?). Modern opamps go to 100kHz.

Opamps don't need a negative supply if their inputs are biased correctly.

again, am learning every day! a last favor: any part names? which Dual Op-amp yould use now? an op-amp with the features you'r talking about..?

 

[audioguru]

I use the inexpensive, low noise, low distortion and wide bandwidth TL071 single, TL072 dual and TL074 quad opamps invented by Texas Instruments but are made by them and also by many other semiconductor manufacturers. They have FET inputs so don't have bias current. They have been used in many home entertainment products for about 25 years.

There are much better opamps available at higher cost but they sound the same as these. These have 0.003% distortion. The better ones have only 0.00008% distortion. The better ones are OPA134, OPA2134 and OPA4134 also made by Texas Instruments.