CD40106B Schmitt Inverter Relay Drive: Why Does My Relay Buzz and LED Flicker?

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#1 21681942 25 Feb 2019 02:22

Graham Rounce Graham Rounce

Anonymous
Post #1
21681942 25 Feb 2019 02:22

Hi - Some might recognise this as a continuation of my prob with schmitt-triggering. With your help I got over that fence, thanks, but then ran smack into another one
I used a CD40106B hex schmitt inverter, as in the pic, and everything seemed fine.

(a) When the input voltage is below about 2V, the other led comes on and the BD139 conducts.
(b) When the input voltage is over about 3V, one led comes on and the BD139 conducts.
When I replace R9 with a relay (with a diode across it), (a) above still works fine, with the relay clicking nicely, but with (b) the led flickers and the relay buzzes until the voltage is high enough, when they both settle. It's as if the schmitt action of that section of the CD40106B isn't happening.
Stop press! To add insult to injury, while checking the behaviour for this post, I found that now when I replace the relay with the resistor again, it no longer wotks fine. The led flickers.....

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#2 21681943 25 Feb 2019 02:47

David Ashton David Ashton

Anonymous

Post #2
21681943 25 Feb 2019 02:47

Graham on the face of it this should work, but there are a couple of reasons it might not... It seems as if you have fried your CD 40106 so replace that and get the circuit working again with the resistor R9 before you go further.You say you put a diode across the relay (the coil I hope!) which would have been my first suspect. Back EMF from relays has destroyed more circuits than I have had hot dinners.....Your wiring of the two transistor emitters feeding the BD139 is a little unconventional but should work well enough. I would suggest putting a diode in series with R5 and R6 (pointing towards the transistors) to eliminate any interaction between them. How did you arrive at the values for R5/6? I would use higher values. Also, your 100 ohm limiting resistors for the LEDs seem a bit low - at 5V with a red LED you'd get around 30-35 mA, a bit much. White LEDs would be ok - they need 3V or more so you'd only get around 8 mA which would be fine.
So I would increase R5/6 to 2.2K or more and R7/8 to maybe 270 (for red/green LEDs) if you're using a 5V supply. If you're using 12V use 10K for R5/6 and 1K for R7/8. Let us know how you go.
#3 21681944 25 Feb 2019 02:59

Aubrey Kagan Aubrey Kagan

Anonymous

Post #3
21681944 25 Feb 2019 02:59

There are several issues off the bat.
1. Weird operation of any CMOS part can happen if the unused inputs are undefined. Your diagram only shows 2 inputs used, leaving 4 floating. Connect all unused inputs to Gnd (or VDD).2. Your LEDs shouldn’t work at all based in the Circuit drawing- they are the wrong way around.3. I must admit that I have not ever seen your wired OR function on the base of the BD139, but I guess it should work.4. You don’t say what your supply voltage is, but using 5V the current through each LED (if they were the right way around) would be (5-1.2 (LED)-0.6-0.6(2 base emitter junctions)0/100=26mA which is high. I would aim for about 10mA. Change the 5 to whatever voltage you are using to get to the correct value for the resistor.5. If you use the relay you should use a diode connected across the coil (cathode to the supply) to prevent the back EMF from the relay damaging the BD139
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#4 21681945 25 Feb 2019 04:00

Aubrey Kagan Aubrey Kagan

Anonymous

Post #4
21681945 25 Feb 2019 04:00

It seems David was posting as I was composing- I didn’t see his message until I was done. The suggestion of diodes in series with R5 and R6 is in fact very prudent. When either of the BC547s is on, the base emitter junction of the other transistor is reverse biased. Offhand Iam not sure what the maximum.is, but traditionally it is quite a low voltage and it could destroy the transistor. I wouldin fact place the diodes in the emiiter line on each transistor with the anode connected to the emitter of theBC547.
I just checked the reverse voltage breakdown and it is 6V so if you are working with a 5V supply, or a little more you should be OK without the diodes.
#5 21681946 25 Feb 2019 05:26

Graham Rounce Graham Rounce

Anonymous

Post #5
21681946 25 Feb 2019 05:26

Hi, and thanks again. I'll work through all that and come back. Do we know why the CD40106 blew up? I'll check that diode across the relay coil.
The supply is 5V.
Yes, all the unused inputs of the CD40106 are grounded.
Yes, the leds are the wrong way round on the diagram! :(Thanks. I'll be back.
#6 21681947 25 Feb 2019 09:03

David Ashton David Ashton

Anonymous

Post #6
21681947 25 Feb 2019 09:03

Aubrey - great minds think alike (or fools never differ, not quite sure which.... I really couldn't find any reason for the failure - I presume it is the 40106 though Graham has not confirmed that fully. In the specs the 40106 can drive 10ma per output so though I recommended he increase the resistors on the outputs, it should have coped OK as it is shown. And as the emitters of both BC547s are on the base of the BD139, they should not go above 0.6V which is well within the reverse BE voltage limit even if the bases are grounded. As the problem started when Graham put the relay in, I'd guess that has some bearing on it, maybe the diode was not properly connected (I've done that :-)Graham - look forward to your further feedback.
#7 21681948 26 Feb 2019 02:05

Graham Rounce Graham Rounce

Anonymous

Post #7
21681948 26 Feb 2019 02:05

Hi - I've replaced the CD40106, and done everything suggested. [See the new pic.]I think that initially it was ok, with the leds going on and off cleanly. But I can't be sure, because shortly one of them started flickering again! [See the video, if I can upload it.] [Hmm, apparently, I can't ]

Anyway, I suppose that rules out the relay! (The diode across which was ok.)I'm seriously starting to think about replacing the lot with a PIC! But now I've got this far... Cheers, Graham.
#8 21681949 26 Feb 2019 02:08

Graham Rounce Graham Rounce

Anonymous

Post #8
21681949 26 Feb 2019 02:08

PS - CD40106 unused inputs all grounded.
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#9 21681950 26 Feb 2019 02:22

Aubrey Kagan Aubrey Kagan

Anonymous

Post #9
21681950 26 Feb 2019 02:22

Of course there is the possibility that your op-amp is oscillating which could cause the flicker. Easiest to tell if you've got a 'scope- but IIRC you don't. Try decouplng the supply to the op amp and the CD40106, but especially the former. Try adding .0.1uf and also on the refernce inputs. Try it also in the "IN", and also try 1000pF across R2 and R4- see if any of that makes a difference.
You may want to try an LED in place of the relay and see if it flickers at the same time as the other LED.
#10 21681951 26 Feb 2019 03:12

Graham Rounce Graham Rounce

Anonymous

Post #10
21681951 26 Feb 2019 03:12

I'll try putting all those caps in - not so easy on a plugboard, but never mind. Speaking of which, it is a bit of a rat's nest. Is that likely to be a prob? It's all lowish impedance, I think?
I do have a scope. Where to attach it - I guess on the opamp outputs for a start?
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#11 21681952 26 Feb 2019 04:14

Aubrey Kagan Aubrey Kagan

Anonymous

Post #11
21681952 26 Feb 2019 04:14

GrahamIs that likely to be a prob?
No- unless wires are waving in the air or broken. However, since it is plug board you can unplug the inputs to the 40106 and set either one high and watch if the LED flickers- obviously doing that removes the op-amp from the equation.
#12 21681953 26 Feb 2019 09:28

David Ashton David Ashton

Anonymous

Post #12
21681953 26 Feb 2019 09:28

Graham, as Sherlock Holmes would say, when you have eliminated the probable, whatever remains, however improbable, must be the cause.You have not said much about your power supply here except that it is 5V. If it fluctuates, especially when the 100R resistor or relay is drawing current, it will alter your threshold voltages and cause exactly this problem.- is it from batteries? They can be notoriously high impedance.- is it regulated? If not think about doing so.- do you have any large capacitors (I'd suggest at least 470 uF) across the power rails?I have had a similar circuit with a 555 timer do very similar things because of similar problems.....Let us know if any of the above helps
#13 21681954 29 Jun 2019 12:49

Larry Normand Larry Normand

Anonymous

Post #13
21681954 29 Jun 2019 12:49

I have some issue1. Weird operation of any CMOS part can happen if the unused inputs are undefined. Your diagram only shows 2 inputs used, leaving 4 floating. Connect all unused inputs to Gnd (or VDD).2. Your LEDs shouldn’t work at all based in the Circuit drawing- they are the wrong way around.3. I must admit that I have not ever seen your wired OR function on the base of the BD139, but I guess it should work.4. You don’t say what your supply voltage is, but using 5V the current through each LED (if they were the right way around) would be (5-1.2 (LED)-0.6-0.6(2 base emitter junctions)0/100=26mA which is high. I would aim for about 10mA. Change the 5 to whatever voltage you are using to get to the correct value for the resistor.
#14 21681955 02 Jul 2019 11:19

Larry Normand Larry Normand

Anonymous

Post #14
21681955 02 Jul 2019 11:19

I have some issue
1. Weird operation of any CMOS part can happen if the unused inputs are undefined. Your diagram only shows 2 inputs used, leaving 4 floating. Connect all unused inputs to Gnd (or VDD).
2. Your LEDs shouldn’t work at all based in the Circuit drawing- they are the wrong way around.
3. I must admit that I have not ever seen your wired OR function on the base of the BD139, but I guess it should work.
4. You don’t say what your supply voltage is, but using 5V the current through each LED (if they were the right way around) would be (5-1.2 (LED)-0.6-0.6(2 base emitter junctions)0/100=26mA which is high. I would aim for about 10mA. Change the 5 to whatever voltage you are using to get to the correct value for the resistor.Hello, i really need your help!Anyone ?
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Topic summary

✨ A circuit using a CD40106B hex Schmitt inverter to drive a BD139 transistor and relay exhibits relay buzzing and LED flickering issues. When the input voltage is below ~2V, one LED and the BD139 conduct correctly; above ~3V, the relay buzzes and the LED flickers until the voltage stabilizes. Replacing the relay with a resistor initially resolved the issue but later caused flickering again. Key troubleshooting steps include verifying the diode across the relay coil to suppress back EMF, grounding all unused CD40106B inputs to prevent undefined CMOS behavior, and checking LED orientation and resistor values to ensure appropriate current limiting (targeting ~10mA). The wired-OR transistor base configuration is unconventional but functional. Potential causes for instability include damaged CD40106B ICs, insufficient power supply decoupling, and possible oscillations in the op-amp stage. Adding decoupling capacitors (e.g., 0.1µF and 1000pF) on supply and input lines, ensuring a stable 5V regulated power supply with adequate bulk capacitance (≥470µF), and testing the circuit with LEDs instead of the relay are recommended. The flickering and buzzing likely stem from power supply fluctuations or back EMF damaging the IC, with the relay diode polarity and wiring critical to circuit stability.

FAQ

TL;DR: CD40106B relay buzz/LED flicker is usually drive, supply, or back‑EMF related. Expect ~30–35 mA with 100 Ω at 5 V, and remember: "Back EMF from relays has destroyed more circuits..." Fix with flyback diode, decoupling, and right resistors. [Elektroda, Anonymous, post #21681943]

Why it matters: This FAQ helps hobbyists and engineers quickly diagnose buzzing relays and flickering LEDs when driving loads from a CD40106B Schmitt inverter.

Quick Facts

Tie every unused CD40106 input to GND or VDD to prevent erratic switching and oscillation. [Elektroda, Anonymous, post #21681944]
Relay coil needs a flyback diode; connect cathode to +V, anode to the transistor side. [Elektroda, Anonymous, post #21681944]
At 5 V with 100 Ω, LED current is approx. 26 mA; aim near 10 mA instead. [Elektroda, Anonymous, post #21681944]
Suggested values: 5 V rails → R5/R6 ≈ 2.2 kΩ, LED resistors ≈ 270 Ω; 12 V → 10 kΩ and 1 kΩ. [Elektroda, Anonymous, post #21681943]
Stiffen the 5 V supply with ≳470 µF across the rails to stop threshold wobble. [Elektroda, Anonymous, post #21681953]

How do I stop my relay buzzing and the LED flickering with a CD40106B?

Add a flyback diode across the relay, decouple the IC and op-amp, and use proper LED/base resistors. A weak 5 V supply shifts thresholds and causes chatter. Start by testing with an LED load instead of the relay to confirm stability. “Back EMF from relays has destroyed more circuits...” so verify the diode’s polarity. [Elektroda, Anonymous, post #21681943]

What causes a CD40106B output to chatter near the threshold?

Floating inputs, insufficient decoupling, or a sagging supply can push the input around the hysteresis. Tie every unused input to a defined level, add 0.1 µF at each IC’s Vdd–GND, and a bulk electrolytic on the rails. Replace the relay with a resistor or LED to isolate supply effects. [Elektroda, Anonymous, post #21681944]

Do I need a diode across the relay coil, and which way around?

Yes. Place the diode directly across the coil: cathode to the positive supply, anode to the transistor/low side. This clamps the coil’s back‑EMF and protects the driver transistor and logic from voltage spikes that cause buzzing or damage. [Elektroda, Anonymous, post #21681944]

Could my CD40106B be damaged—what likely killed it?

A miswired or missing flyback diode lets the relay’s back‑EMF punch the output stage or the driver transistor, propagating stress into the 40106. One helper noted the 40106 can handle about 10 mA per output, so the relay event, not LED load, is suspect. [Elektroda, Anonymous, post #21681947]

How should I size the LED resistors at 5 V?

With 100 Ω and a red LED, current lands around 26 mA. Target about 10 mA to reduce loading and noise; increase the series resistor accordingly. This also prevents supply droop that shifts the Schmitt thresholds and induces flicker. [Elektroda, Anonymous, post #21681944]

What are good resistor values for the BC547 drivers and LEDs?

For 5 V rails, use about 2.2 kΩ feeding the BC547 bases and ~270 Ω for LED series resistors. For 12 V systems, move to ~10 kΩ for bases and ~1 kΩ for LED resistors. These values limit current and reduce interaction. [Elektroda, Anonymous, post #21681943]

Is my breadboard “rat’s nest” causing the flicker?

Usually no, unless wires dangle or break. To rule it out, unplug the op‑amp and manually force the CD40106 inputs high/low; watch whether the LED still flickers. This isolates the logic and proves if the board layout is at fault. [Elektroda, Anonymous, post #21681952]

How do I check for op‑amp oscillation that makes LEDs flicker?

Probe the op‑amp outputs with a scope. Add 0.1 µF decoupling at the op‑amp and CD40106, and try small capacitors on the reference network (e.g., 1000 pF across R2/R4). Also try a small cap on “IN” to tame noise. [Elektroda, Anonymous, post #21681950]

What bulk decoupling should I add to stop threshold wobble?

Place at least a 470 µF electrolytic across the 5 V rails near the load and logic, plus local 0.1 µF ceramics. This reduces supply impedance so relay or LED current pulses don’t modulate the CD40106 thresholds. [Elektroda, Anonymous, post #21681953]

Are my unused Schmitt-trigger inputs allowed to float?

No. Floating CMOS inputs pick up noise and can latch unpredictably. Tie every unused CD40106 input to ground or VDD. This single change often eliminates weird flicker and prevents extra supply current. [Elektroda, Anonymous, post #21681944]

Is wiring two BC547 emitters together into a BD139 base okay?

It is unconventional but workable. To protect the BC547s, consider diodes to prevent reverse base‑emitter stress. The BC547’s reverse B–E breakdown is about 6 V; staying under this avoids damage when the other path drives. [Elektroda, Anonymous, post #21681945]

How can I quickly isolate whether the relay or logic is the problem? (3‑step How‑To)

Replace the relay with an LED and resistor; check for flicker.
Add flyback diode to the relay; re‑test with relay.
Add 0.1 µF at ICs and ≥470 µF on rails; retest thresholds. [Elektroda, Anonymous, post #21681950]

My supply is 5 V—could it still be the culprit?

Yes. High‑impedance or unregulated 5 V rails can droop when the relay or a 100 Ω load pulls current. That shifts thresholds and causes chatter. Use regulation and add a ≥470 µF capacitor across the power rails to stabilize the system. [Elektroda, Anonymous, post #21681953]

Can I skip discrete fixes and use a PIC instead?

You can, but the thread shows the discrete path works once decoupling, flyback, and resistor values are correct. Fixing fundamentals gives immediate results and keeps the design simple and robust. [Elektroda, Anonymous, post #21681948]

What is a Schmitt trigger inverter (CD40106B)?

It is a CMOS inverter with hysteresis that cleans noisy signals. It switches low near one threshold and high near another, reducing flicker around boundaries. Proper supply decoupling and defined inputs are still essential for stable behavior. [Elektroda, Anonymous, post #21681944]