NE555 pulsed IR emitter 36 kHz : strange spurious signal at the ouput

Thread Starter

Philexium

Joined Oct 15, 2017
85
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
 

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Thread Starter

Philexium

Joined Oct 15, 2017
85
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 ?
 

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KeithWalker

Joined Jul 10, 2017
3,187
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.
 

Thread Starter

Philexium

Joined Oct 15, 2017
85
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
 

Audioguru again

Joined Oct 21, 2019
6,785
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.
 

AnalogKid

Joined Aug 1, 2013
11,208
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:

1681832448944.png

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
 

Audioguru again

Joined Oct 21, 2019
6,785
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.
 

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Thread Starter

Philexium

Joined Oct 15, 2017
85
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
 

Thread Starter

Philexium

Joined Oct 15, 2017
85
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.
 

Thread Starter

Philexium

Joined Oct 15, 2017
85
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 ?
 
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