Power Supply fluctuations

Hi all,
I've made a Remote Controlled switch using TSOP1738, a Timer 555 and a CD4017. The working is this :

1) The 555 timer is set to monostable mode with the trigger input connected to the output of TSOP1738
2) Whenever pulsed IR falls on the TSOP1738, it triggers the timer.
3) The timer drives the clock of CD4017.
4) The CD4017 is wired in such that way that the output appears as follows Q0, Q1 then again Q0, Q1 and so on
5) The output of the CD4017 is hooked to a relay via a driver transistor.

Now when I power on and press any button on the remote. The output toggles, as expected.

The problem starts when any appliance on the house is switched on/off... for eg: Fan, lights etc, the output toggles !! First I thought the cheap chinese AC-DC SMPS circuit is faulty, so I replaced is with a simple most basic linear power supply with a 12V transformer, bridge rectifier, 1000 uF capacitor and 7809 regulator... There's also a 5V zener for the TSOP1738...

But the problem still occurs. Any solution ?

Is the problem caused by a breadboard with rows of contacts and jumper wires all over the place picking up interference? Or is it caused by the TSOP1738, 555 or 4017?
You didn't read the datasheet of the TSOP1738. It shows a resistor and capacitor filter feeding 5V to it and says, "recommended to suppress power supply disturbances".

The TSOP1738 is connected as recommended in the datasheet. The circuit is soldered on a veroboard. I can't find out the exact source of the problem... but it occurs whenever some switching takes place in the house.

The basic problem is that you are using the carrier and not any specific code. ANY key on the remote with the right carrier will toggle the CD4017 and you will be happy. But you must decode the key sequence from the remote and act accordingly. Just detecting the carrier is subject to noise and IS NOT THE recommended way. You need to decode a particular sequence.

I think I didn't describe my problem clearly. The circuit works fine when I'm using battery power. It only starts malfunctioning when I'm using an AC to DC converter to power it up.

If what you describe is true, then the photodetector plays no role. Your circuit design is noise sensitive. When you power by battery, do you mean you use battery power for receiver, then there is no noise trigger?

Conducted susceptibility, to any incoming disturbance.
A simple envelope detector can easily be spoofed. A
detector that insists on some more particular attribute
(like a PLL lock detect) will be more trash-rejecting.
So might (perhaps) a power supply with better common
mode filtering on the inlet side, but this may be only a
matter of degree, RF may slide on by it.

You said you used a 5V zener diode for the IR receiver but did not say a resistor and capacitor as shown on the datasheet. A capacitor filters much better than a zener diode and nobody makes a 5V zener diode (maybe it is 5.1V and maybe its current is too low for it?). Please post your entire schematic of the circuit that uses a transformer. Please post the part number of the "5V" zener diode.

Sorry for the late reply...

@Audioguru : Sorry I'm not being able to provide you the circuit diagram because of my horribly slow internet speed doesnt allow me to upload images, I'm using a text only version of the website. But, I hooked up the TSOP1738 in the following way : a 1k resistor from Vcc to supply pin, a 5.1 V zener diode from supply to ground, with a 10 uF capacitor parallel to the zener.

@C_Mitra : Yes you are absolutely right. When I'm powering the circuit with a battery, the circuit works fine even when the other appliances are turned on and off. But as soon as I power it via a ac-to-dc converter, it works fine when no appliances are switched on/off, but as soon as a turn on say the fan or the TV, the output changes.

@dick_freebird : I didn't get you

Your Zener has Little Rejection of Line Interference.
It would be Better to use a Linear Regulator, such as a 78L05.

I'm using a 7809 to keep the voltage at 9 volts Vcc since I'm usin a 12 Volt transformer for AC to DC conversion. Do I still need the 7805 or is the 7809 sufficient for the job ?

Partly Depends on What Capacitors your using for Filtering?
But I would still use a 78L05.

And your TSOP1738, should have a .1uf Cap, connected Directly across its +/- terminals.

I'm using a 1000uF capacitor after the bridge rectifier... Will use a 0.1uF across the TSOP1738 and post the result.

You forgot to calculate the current in your 5V (?) zener diode. The 1k resistor from 9V has 4 V across it then its current is only 4mA. The TSOP1738 has a maximum supply current of 1.5mA. Then the zener diode gets only 2.5mA but the zener diode might need 20mA to work properly. What is the part number of your zener diode?

I have a graph of the dynamic impedance of zener diodes. You want the dynamic impedance to be very low then it is a good voltage regulator. With a current of 20mA a 5.1 zener diode has a dynamic impedance of 2 ohms and is a good regulator. With a current of only 2.5mA a 5.1V zener diode has a dynamic impedance of 150 ohms and is a lousy regulator. Your zener diode might need a current of 50mA or more to be a good regulator. A 7805 or 78L05 is a much better regulator anyway.

DasPreetam said:

Will use a 0.1uF across the TSOP1738 and post the result.

Click to expand...

You are expected to use 0.1uF ceramic cap as the power supply RF ripple filter; you need to put it between 5V (or 9V; as the case may be) and ground at a place close to the detector. If possible, use a small disk cap.

Tried every damn possible thing... No results.. Went to buy a similar circuit in the market, the shopkeeper said he stopped selling those because the output changes whenever any appliance starts :lol:. So, the problem is everywhere.

So I shelved the project. I tried to make it low cost but failed... Looks like I've to go for an microcontroller based design (preferably Arduino) which will definitely cost more. Thank you for all your support.