Hi,

I have designed a NE555 pulsed IR emitter at 36 kHz.
The circuit is very "classic" and it oscillates correctly.
The problem when I connect the scope at the ouput on pin 3 :
- There is a stong "spurious" signal on the front edge.
I have tried different chips from TI but the problem remains the same.
When I connect a bigger capacitor to lower the frequency the signal is clean.
The frequency is 36 kHz it's not a lot for a NE555 !
Any idea ?

Thank you

 

Are you using a a 10:1 probe with the ground clip connected near the 555 ground pin?

 

Have you adjusted your scope probe for optimum square wave response?

 

Have you adjusted your scope probe for optimum square wave response?

Good evening @KeithWalker

I have enclosed a photo of the probe.
Please note that the second photo noted X10 is with the X10 MAG button on the scope.
It is an old 2245 a Tektro scope and sorry but I don't know how to adjust the probe ...

Do you think the scope could give a false information ?

 

You didn't answer my question about where you put the ground clip on the probe.

 

There is a 1 KHz square wave test signal available, just to the left of the Channel A input BNC connector. Clip the tip of your X 10 probe to it and vary the compensation adjustment on the side of the probe for the squarest leading edge on the viewed waveform.

 

There is a 1 KHz square wave test signal available, just to the left of the Channel A input BNC connector. Clip the tip of your X 10 probe to it and vary the compensation adjustment on the side of the probe for the squarest leading edge on the viewed waveform.

Yes I have adjusted it and the same problem remains

 

You didn't answer my question about where you put the ground clip on the probe.

Soory The ground clip is on pin 1
I am afraid that as I am using a breadboard (Even if it is a very small one) there may be some lead/capacitance effect ?...

 

Of course the breadboard is causing the problem. Its rows of contacts and wires all over the place have inductance.

 

Another possibility is the 36kHz from a compact fluorescent or mains LED light might cause interference to the wiring or have the pulsing light picked up by the diodes.

 

I doubt that the spike as any negative effect on the circuit operation.

 

I doubt that the spike as any negative effect on the circuit operation.

Hi

I was suggested to use a TLC 555 CMOs version of the NE555.
How do you believe it would help ?

 

How do you believe it would help ?

Not likely.

 

If the scope has a bandwidth limit switch, turn it on.

Separate from that, you can eliminate both diodes and one resistor and still get a 50/50 square wave. The circuit is in the LMC555 datasheet, copied here:



This circuit works well with the bipolar 555, but the output is not perfect because the bipolar 555 does not have symmetrical headroom voltages in its output stage. How close to 50/50 do you need?

ak

 

An old fashioned 555 has old TTL logic outputs that cause a "shoot-through" power supply current pulse of almost 400mA (!) each time the output switches. Those high current pulses on a breadboard makes messy waveforms.

A Cmos 555 dont doo dat.

 

If the scope has a bandwidth limit switch, turn it on.

Separate from that, you can eliminate both diodes and one resistor and still get a 50/50 square wave. The circuit is in the LMC555 datasheet, copied here:

View attachment 292385

This circuit works well with the bipolar 555, but the output is not perfect because the bipolar 555 does not have symmetrical headroom voltages in its output stage. How close to 50/50 do you need?

ak

Hi @AnalogKid

Thank you for the schematic !
What is the data sheet where you have found it ?
I have some from TI, Signetics and didn't find it ...
I will test it and keep you posted
Philippe

 

An old fashioned 555 has old TTL logic outputs that cause a "shoot-through" power supply current pulse of almost 400mA (!) each time the output switches. Those high current pulses on a breadboard makes messy waveforms.

A Cmos 555 dont doo dat.

Hi @Audioguru again

Very interesting explanation Well I do need to order some CMOS chips. I will choose the TLC555 from TI
I will test it when I receive them and keep you posted

Thanks again and good evening from France

 

It sounds like you're having some trouble with your NE555 pulsed IR emitter circuit. It's great to hear that the circuit is oscillating correctly, but the spurious signal on the front edge is definitely a concern. Have you tried adjusting the resistance values in the circuit? It's possible that changing the resistance could help clean up the signal. You might also want to double-check your connections and make sure everything is properly grounded.

 

Page 16.

https://www.ti.com/lit/ds/symlink/l...6tisearch%3Dsearch-everything%26usecase%3DGPN

ak

 

Page 16.

https://www.ti.com/lit/ds/symlink/lmc555.pdf?ts=1681810507566&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLMC555%3FkeyMatch%3DLMC555%26tisearch%3Dsearch-everything%26usecase%3DGPN

ak

Hi @AnalogKid

I have tried with the schematic you have provided
As I have not yet found available LMC555 I have tried with the old NE555
But it is nor working fine so I think this design is specific to the CMOS LMC555.

I's difficult to find here available chips.

But the TLC 555 from TI is very similar and available. What do you think ?