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I'm building a Hifi amplifier and I wanted to design a circuit which switches off the power of the amp boards if no music is played for like half a minute. I built it on a demo board but it looks like I made a mistake somewhere, the current is not enough to pull the relay.

circuit

Is my design correct at all? The theory is that music (the source is a phone or a laptop output) is fed into the opamp, the R1 (500 ohm)-R2 (10k) amplifies it and the positive half of the signal (which goes through D6) charges the C2 capacitor. If C2 is sufficiently charged, Q4 starts to conduct and closes the relay on J5-J6. Without music, the capacitor discharges, Q4 no longer conducts, the relay opens. Problem is, the voltage on the relay is very low and doesn't pull. I also tried using a mosfet (BS170) for Q4, same thing happens. Is my design flawed or should I search for hardware problems?

balu
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3 Answers3

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The 741 is powered from 0 volts and +12 volts. This means that the maximum positive output voltage is restricted to about +10 volts (on a good day). This voltage is then fed to an emitter follower (Q2) and the maximum output voltage on the emitter will be about 0.7 volts lower than the base hence 9.3 volts.

This voltage is further reduced by D6 to maybe 8.7 volts and, the final transistor (Q4) emitter output will be lower by another forward diode drop. I'd expect to see about 8 volts feeding the relay on a good day.

If your relay is 12 volt rated on its coil then that is why it likely doesn't operate.

Problem is, the voltage on the relay is very low and doesn't pull.

This doesn't surprise me.

Is my design flawed

Yes it is. Here are other problems: -

  • The 741 is a really poor choice these days and it has been for a couple of decades.
  • The input signal you are feeding in is biased around 0 volts and the lowest supply on the 741 is also 0 volts and this is just beyond the capabilities of this dinosaur and many modern op-amps. You just can't expect any op-amp to work like this.
Andy aka
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  • Are there op-amps that can cleanly reproduce a signal such as audio, if it's centered on one of the voltage rails? – Kaz Jul 14 '20 at 06:41
  • @Kaz there are a few. – Andy aka Jul 14 '20 at 08:28
  • +1 for "really poor choice", but you need to go further back. I was at uni in the early 90s and working on analogue circuits with my sponsorship company. The engineers there considered it a long-obsolete piece of rubbish even back then. So it's at least 40 years since anyone should have considered designing with a 741. – Graham Jul 14 '20 at 08:43
  • @Graham did you follow the link to the "poor choice"? – Andy aka Jul 14 '20 at 09:02
  • @Andyaka Yep - totally agree. I'd just revise your "couple of decades" to more like "4 decades". :) – Graham Jul 14 '20 at 09:28
  • Well I said it's been a *really* poor choice for a couple of decades (the inference being that it's been a "regular" poor choice for significantly longer)!! – Andy aka Jul 14 '20 at 09:32
  • @Andy Ah, that jogs my memory. Yes; there are some op-amps that magically work a bit outside of the power rail region. – Kaz Jul 14 '20 at 19:25
  • @Andyaka Thanks for the explanations. I used the 741 only because I had one lying around. . – balu Jul 15 '20 at 19:13
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Put the relay between Q4 collector and the positive supply. The relay should be rated for 24 volt operation.

With the relay between Q4 emitter and Ground, Q4 is an emitter follower, and its emitter voltage will be about 0.7 volt below the base voltage.

Peter Bennett
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  • I can't see how putting a 24 volt relay in the collector feed will help. It will need other circuit modifications such as connecting emitter to 0 volts and then the delay time circuit on the base is significantly affected etc.. – Andy aka Jul 14 '20 at 09:04
  • I will look into that in a few days. – balu Jul 15 '20 at 19:15
  • Putting the relay between 24V and drain (left the BS170 in the circuit after all) and grounding source worked well. Now the only issue is that if I connect it to a phone jack, it pulls the relay even if the music is stopped. There must be some voltage on the jack even if no music is played. If I unplug it, the relay opens after about a minute. – balu Jul 16 '20 at 18:19
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You'd be better off using an open collector comparator: unlike an opamp it is well behaved for signals exceeding its rails, and doesn't suffer from not being able to swing to the rails.

enter image description here

Choose R1 and R2 to create a threshold voltage, say 0.2V.

When positive swing of audio input reaches that threshold the comparator flips low and yanks the capacitor C to ground. (If C is large then maybe put a small current limit resistor at the comparator output.) This pulls the base of the PNP transistor down and turns it on, connecting rail V1 to the relay coil. Choose V1 to match your relay voltage (5V, 12V, 24V).

When audio stops, comparator goes high, which for an open collector device is an open circuit, so C will slowly charge to V1 through R3 and R4, eventually turning off the transistor.

The relay switches in V2 rail to the amplifier. V2 can be independent of V1.

td127
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  • This would need buying some parts (I don't think I have a comparator in my stock right now) and a bit more work, but definitely worth considering. – balu Jul 15 '20 at 19:17
  • Well, if you're hellbent on using the 741, an opamp can be used as a comparator (not a great one, but fine for this application), and you can fake the open collector output with a diode: cathode facing opamp output, anode to R4/C node, no current limit resistor necessary. The capacitor will only discharge to 2V or so, but that's good enough to turn on the transistor. And the opamp is unlikely to work without raising the threshold voltage to somewhere in the 1 to 2V region. – td127 Jul 15 '20 at 21:38
  • I checked Peter Bennet's solution and it kind of works now. Putting the relay between 24V and drain (left the BS170 in the circuit after all) and grounding source worked well. Now the only issue is that if I connect it to a phone jack, it pulls the relay even if the music is stopped. There must be some voltage on the jack even if no music is played. If I unplug it, the relay opens after about a minute. – balu Jul 16 '20 at 18:22