NE555 IR Repeater

[Gayan Soyza]

I designed an IR Repeater using NE555.

The problem is without the Load (IR LED) its functioning.When I connect the load (IR LED) it will trigger all the time without a remote.In other words pin 4 is high when I connect the Load (IR).

 

[Boncuk]

Did you measure the collector voltage of T2 with and without output signal from the IR receiver?

 

[user_88]

... Results from a simulation on LTSpice:

The input which was used for this simulation was a 1 kHz frequency, with 100 µsec pulse width. ... Used 2N2222 instead of D400 transistor at the output. ... Used a visual LED instead of the IR version.

An interesting observation was that the C1 capacitor ... originally 0.001 µF ... appears to cause a sawtooth effect on the output current waveform of the circuit ... the current going through the output diode. The sawtooth amplitude of this waveform is about 50% of the total amplitude. This sawtooth waveform also has a higher sub-frequency, within the output pulse.

Could this particular event be related to your problem?

Changing the value of C1 to 0.1 µF appears to correct the output sawtooth deviation. The output is essentially a simple square wave ... following the input very closely.

... additional comment:
I think that the sawtooth waveform was actually observed at at pin 4 of the NE555. However, at the same time, there is an obvious abnormality in the current through the output LED. If you take an oscilloscope and probe pin 4 at a high enough resolution, you should be able to determine whether your original circuit has a similar defect.

 

[Gayan Soyza]

Thanks mates for your kind observation.

@ user_88
Changing C1 to 0.1 µF will make the frequency to 38hz which will never operate IR modules.The carrier needs to be 38Khz pulses.

@ Boncuk

I measured T2 collector voltage

With the IR module placed the voltage is 0V.

 

[ronv]

If it works without the IR emitter is it possible you have a feedback problem where the output IR is getting back into the input receiver? You could make the IR emitter an LED to see, or cover the receiver real good. They can be very sensitive.

 

[Boncuk]

Hi Gayan,

I have also simulated your circuit using a continuous input square signal of 38KHz at 4.8V output voltage from the simulated IR-receiver which is done using a function generator.

The output signal follows exactly the input signal as you can see by the scope screenshot.

Extensive simulation shows the timer circuit runs as an astable multivibrator if the reset input voltage exceeds 1V (pretty precisely).

With the base of T2 floating this will be the case causing 1.3V at the collector which connects to the reset pin of the timer IC, hence not resetting it!

So you must force the base of T2 high enough to pull the reset pin low if there is no reception of the IR-receiver.

If you're using a series resistor with the power (+V) of the receiver just short it out and check what's going on.

Scope presentation: channel1 (yellow) -> input signal from receiver, channel2 (blue) -> signal at the base of T1, channel3 (violet) -> collector voltage of T1, channel4 (green) -> not used.

Next time I want to read "success".

Kind regards

Boncu

 

[Gayan Soyza]

Hi guys thanks for your input.I solved that load thing.Boncuk thanks for your extreme help.

I use these values.
R1 6.8K,R2=15K,C=.001 (102) In theory the frequency is 39KHz.But the scope shows almost half way mark 19KHz........

That's the latest problem.I also increased the supply to 9V Also generated 5V to TSOP1738.The circuit works but not in required frequency range.

I also notices there are some values it doesn't work with with NE555.

 

[user_88]

... from 555 Data Sheet

From one of the application circuit paragraphs:

"Adequate power supply bypassing is necessary to protect
associated circuitry. Minimum recommended is 0.1μF in parallel
with 1μF electrolytic"

... might be worth looking at.

 

[Gayan Soyza]

Nop, I also already added decoupling caps 0.1uF.

IR modules will accept 50% duty cycles.But with my RRC values it gives 60% duty cycle.

May be R1 < R2 by ten times.

 

[user_88]

Looking at the LM555 Data Sheet ... Figure 6 .... Free Running Frequency
It looks like the 0.001 µF capacitor is at the extreme range of the chart.

A few trial simulations produced an alternate set of values:
Ra=1.1k
Rb=1.3k
C=0.01 µF

Resulting in an approximate free running frequency of 38 kHz ...

... Something like 64% on duty cycle for these values.

 

[Gayan Soyza]

Frequency won't Increase above 20KHz

I'm still struggling with this issue..

*That capacitor extreme range (0.001uF) can be a problem.
*The vero board (track board) can be another problem to add capacitance & cause to reduce the frequency.