WEEKEND PROJECTS FOR QRPERS
By George Dobbs G3RJV
First offered as part of the Four Days In May Symposium at the 1999 Dayton Hamvention.

Audio-derived S-Meter

When operating a receiver, there is something re-assuring about having a little meter needle dancing up and down in sympathy with the signal strength. The fact that it may not be doing much in the way of objective measurement seems to be of little importance. The more experienced operator knows what an S7 signal sounds like on the receiver and also tends to doubt anyone who says "you are S7 on the meter". Many S-Meters only offer subjective readings but they do look nice on the front panel.

So I begin this little project with honesty. The meter described here has no objective accuracy at all, but it does indicate the relative audio output of the signal being tuned. Its chief advantage is cosmetic - it makes the front panel look better and gives the pleasing effect of seeing the signal as well as hearing it. It is based on a circuit by HE9VXB

Since the circuit has to connect to the existing receiver audio stages, it is important that it does not offer a significant load to these circuits on the point of contact. Thus the input stage for the S-Meter offers a high impedance. A relatively high value pre-set potentiometer feeds the signal to a FET stage. This arrangement is unlikely to have much effect wherever it is connected, apart from sampling the audio signal. The stage really acts as an impedance transformer.

The capacitor, C4, couples the audio signal to a relatively high gain audio amplifier stage. This uses a bipolar device. Once again this could be almost any generic NPN transistor. I have a stock of the BC182 so that is what I used. If, in practice, the overall gain of the S-Meter seems a little low, try increasing R4 to around 100K ohms.

The capacitor, C6, feeds the audio signal to a detector circuit which converts the audio signal into a relative DC signal. D1 and D2 act as a voltage doubler detector to drive the meter from the load, R9. The diode types are also uncritical. Mine are the popular 1N914, or they may have been the 1N4148 - who knows!

C8, a 4u7 capacitor, smoothes out the movement of the voltage. This may need a little experimentation. If you want the needle to give steady readings, increase the value of C8, if you like it to dance around a little, decrease C8. I found that 4u7 was a good compromise value.

The meter is one of those small edge-wise CB type S-Meters so often found at reasonable prices. They usually have a full-scale deflection of some 200uA. Any meter with a similar full-scale deflection would do the job.

The question remains - where to connect the S-Meter? I tried the circuit on several receivers, from simple direct conversion receivers to commercial superhets.. The most appropriate place seems to be at the top end of the audio gain control. Not the slider of the control as this would obviously change the reading as the audio gain control was used.

In most cases this will give enough audio signal to produce a useful range of readings. However it is possible to connect the input of the meter further down the audio amplification circuitry. There is no reason why it cannot be connected directly on to the output at the loudspeaker or headphone socket. The only answer is to try it and brighten up the front panel of that simple receiver.

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