Problem with MUX

[ruzfactor]

I will try to get one LM393. In the mean time I have think of something. Why cant I use a CMOS Inverter solve the problem?
One more thing, I have a LM324.

 

[ruzfactor]

I have added a 74HC04 in the circuit and the MUX now works fine. But Will it be a problem later?

 

[audioguru]

I have added a 74HC04 in the circuit and the MUX now works fine. But Will it be a problem later?

The input threshold voltages of a 74HC04 are very close to the output high and output low voltage at the output of the lousy old 741 opamps and the circuit might fail by itself or fail if the 74HC04 or an opamp are replaced.

 

[ericgibbs]

I have added a 74HC04 in the circuit and the MUX now works fine. But Will it be a problem later?

hi.
Have a look at these two links and see why I am concerned about the logic levels.
Remember you are going to be controlling electrically 'noisy' motors close to the logic, so what do you think is best...

http://www.interfacebus.com/voltage_threshold.html

http://www.interfacebus.com/voltage_LV_threshold.html

 

[ericgibbs]

I will try to get one LM393. In the mean time I have think of something. Why cant I use a CMOS Inverter solve the problem?
One more thing, I have a LM324.

hi,
The LM324 quad OPA has a Vout low of about 200mV, so it would be a better choice than the obsolete 741..

 

[audioguru]

hi,
The LM324 quad OPA has a Vout low of about 200mV, so it would be a better choice than the obsolete 741..

Its output voltage is 0.2V only when its output current is very low like the input of a Cmos IC. If it tries to drive the high current of a TTL gate then its output low voltage is very close to the TTL threshold voltage of 0.8V.

 

[ericgibbs]

hi ruzfactor,

As a quick check I have built your circuit using the LM324 and 74LS151.

Used a number of different LM324 and 74LS151 ic's.

Highest values at the output of the LM324, +0.65V low and 3.7V high.

If you add a 2K2 pulldown resistor on the outputs of the LM324 you will get
a low of +0.5V and a high of 3.6V, which is good enough to drive the LS151

If you cannot get the LM393, try the LM324 it will be much better than the 741.

I would strongly recommend that you try to source a LM393 as it will give a more reliable operation of the circuit.

 

[ruzfactor]

Hi ericgibbs
I have got a LM393. Let's see if it works. And there's a small mistake in the circuit schematic. The inverting input should be the reference voltage.

 

[ruzfactor]

It worked! LM393 gives almost zero output and it gives +2.45V on high at O/P.

 

[ericgibbs]

It worked! LM393 gives almost zero output and it gives +2.45V on high at O/P.

hi,
Thanks for letting us know.

Keep us posted on the project..

EDIT:
hi ruzfactor.
Be sure to fit a 3K3 resistor from the outputs of the LM393 upto +5V..

 

[Roff]

It worked! LM393 gives almost zero output and it gives +2.45V on high at O/P.

That sounds like you did not use pullup resistors on the outputs of the LM393, as Eric suggested. Since the LM393 has open collector outputs, you are effectively letting the 151 inputs float to a logic high, which is poor practice. Use resistors in the range 4.7k to 10k (value not critical) from each 393 output to Vcc.

 

[ruzfactor]

Thanks a lot. I have added a 10K resistor from both O/Ps to the Vcc.

 

[ruzfactor]

Someone please help me out! I don't understand this problem: When the LDR isn't detecting any light inverting input gives around +3V (reference voltage) and the non-inverting i/p gives about +4.8V. but when i tune the POT(100K) this voltage doesn't fall below +4.6V. What could be the problem?? Both the comparators are giving such value.

 

[Roff]

If you don't have any light on the LDR, its resistance will be much more than 100k, so the voltage on the noninverting input will not change much when you change the pot. Illuminate the LDR with the desired light level, then adjust the pot until the comparator output changes.

 

[ruzfactor]

hi Roff

Can you please suggest me what's the best combination of resistance and voltage for making the LEDs brighter? I have used six LED (transparent which emits red light) but the light is not bright enough. that's why my LFR is not working satisfactorily.

 

[ericgibbs]

hi Roff

Can you please suggest me what's the best combination of resistance and voltage for making the LEDs brighter? I have used six LED (transparent which emits red light) but the light is not bright enough. that's why my LFR is not working satisfactorily.

hi ruzfactor,
Will you please post a simple sketch showing how and where the LED's are connected, also what type of LED.

I have looked back thru the posts and I cannot see a circuit for the LED's.

 

[ruzfactor]

Here's a picture of how i placed the LEDs:

I have connected the LEDs with a common wire. The wire is connected to +6V with a 320ohm resistance (100 and 220 in series). Should I add more LEDs? I have run my LFR over a black surface with track made of white tape.

 

[ericgibbs]

Here's a picture of how i placed the LEDs:

I have connected the LEDs with a common wire. The wire is connected to +6V with a 320ohm resistance (100 and 220 in series). Should I add more LEDs? I have run my LFR over a black surface with track made of white tape.

Hi,
Are you saying you have connected 6 Red LED's in series like the drawing.???

Each LED has a forward voltage drop of 2Volts.

OR...

the second drawing. Which is it.?

 

[ericgibbs]

hi,
Look at this drawing.

Three pairs of 2 LEDs [Red].

 

[audioguru]

The LEDs are too dim because their current is too low.