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A system I built uses a commodity siren for audio feedback. In production it's outdoors and 50m from the operator and the volume level is just right. On my bench it's deafening. The tone generator board lacks provision for volume adjustment so I suppose I'll have to attenuate the output.

The whole system is nominally 30W. The resistive component of the horn seems to be about 6Ω. Quite likely this is nominally an 8Ω device.

All the pots I have lying about are for much higher resistances; it seems to me I need to add quite a small resistance. Would I be better off trying to adjust bias on one of the the amplifying transistors? How would you go about adding volume control?

I have already taken the amusingly prosaic step of putting a sock in it. When working on other aspects I simply disconnect the horn but for wiring and system integration checks there's no getting away from it.

For those who like to have the background info...

  • The module at left is a 30A relay with optoisolator for low trigger current.
  • The horn tone generator is the centre module in this photo.
  • Immediately above the tone generator is a garage remote relay module that connects an Arduino digital input to GND when the fob button is pressed.
  • At right is an Arduino Nano socketed on a screw terminal board.
  • The round thing at top is a piezo buzzer providing audio feedback on the bench.

Seen in profile along the bottom wall we have

  • A buck converter adjustable step-down module providing 5V for the Arduino
  • A buck converter adjustable step-down module providing 6V for the servomotor
  • A Freetronics microphone module used to detect engine run state so the controller knows about manual engine start/stop.

Yes it really is all held together by industrial Velcro. What started out as a convenience in a rapidly evolving prototype turned out to be an outstanding shock-mount.

The controller and a big diesel water pump are 100m away down at the river. Due to terrain they are barely audible from outside the house and completely inaudible inside. The neighbour's dogs bark at everything else but the pump doesn't seem to bother them.

Riverside pump

Peter Wone
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  • Consider a FET as a pot controlled voltage controlled resistance using a FET with 1% of the load and same rated power with heat sink. Thresholds are typically 1/3 of decent ON resistance like 2-4V for minimum and 3x threshold for max power. You can remote control the gate voltage with a big cap for noise filtering. The control current is near zero. So any pot will do from Vth to >=3xVth or 3 to 10V depend on choice of power Nch FET <= 60mohm with heatsink. But no need for the sock just replace with remote controlled FET on low side. use twist pair or STP cable. – Tony Stewart EE75 Aug 24 '20 at 03:44
  • You trying to scare the digs or the neighbours? – Tony Stewart EE75 Aug 24 '20 at 03:48
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    Why is this excellent suggestion in the comments and not an answer? – Peter Wone Aug 24 '20 at 08:29
  • Re: "Put a sock in it." Many years ago, one of our customers sent us a bunch of two way radios to be modified. We tested every radio for proper function when we received them. Some joker in the customer's shop had turned the volume all the way up. It was a pushbutton system. To turn the volume down, you had to first turn the radios on. At power on, the radios played (loud) and long "gong" sound. The first nearly blew my ears out. Thereafter, I put the speaker face down on a cushioned chair and sat on it at power on. (continued) – JRE Aug 24 '20 at 08:41
  • We had one complete wiring harness for the radios. The connectors were all molded to the cables - modification wasn't allowed because the harness belonged to the customer and had to be returned in original condition. I spent the better part of a day doing "connect radio, sit on speaker, turn on radio, turn down volume, turn off radio, do the next." The speaker was only driven by a 12W amplifier, but that was the continuous power rating. My backside was numb after turning down all the radios. – JRE Aug 24 '20 at 08:46
  • @JRE that's the sort of priceless tale that ought to be in the preface to an electronics textbook – Peter Wone Aug 24 '20 at 08:48
  • @TonyStewartSunnyskyguyEE75 your comment is obviously the _right_ answer so I don't expect anyone else to add much. Put it in an answer so I can accept it. – Peter Wone Aug 24 '20 at 23:10
  • Yah JRE those were the days. Your bum was numb , my brain was nummer , (sp?) testing my video 12 method scrambler descrambler design on the same laser disc with Smokie and the Bandit over and over and over – Tony Stewart EE75 Aug 24 '20 at 23:56
  • IOS cant copy my text, just as a link. And I’m deadbeat with 10 bags of leaf branches with my repaired hedge trimmer gearbox – Tony Stewart EE75 Aug 25 '20 at 00:00
  • You don't have to do it right _now_. – Peter Wone Aug 25 '20 at 00:02

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