Add S-meter to VK3YE Audio AGC

Curt WB8YYY has been pleased with the  somewhat unusual VK3YE AGC circuit, that uses a LDR and LED pair,  as it nicely removes the top of large signals.

VK3YE has suggested measuring current in parallel with the LED that drives the AGC action, but he found this gave little indication of relative signal strength.  In fact, it works much better measuring current in series with the LED.

Curt is using a small meter movement supplied by Sunil, about 250 uV peak current.  A shunt resistor across the meter is necessary since the LED current at peak is at least 20 mA.

The approximate value of the shunt resistor can be found using the formula Rsh = ( Im x Rm ) / Ish.  Rm was unknown but Curt was able to measure it with his DVM at around 500 ohms.  Inserting the two known values gives a shunt resistance value of 6.25 ohms.

Curt found a resistor of around 5 ohms and it working nicely.

He says “Its not a real S-meter response – let’s call it a signal strength meter.”  It can discern signals from approximately S5 to S9.  for signals that do not result in meter movement, the LED itself could be observed to sense signal strength – but the meter represents a nice touch.

 Reference

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