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LF: Re: meter problem (2)

To: <[email protected]>
Subject: LF: Re: meter problem (2)
From: "Peter Martinez" <[email protected]>
Date: Mon, 15 May 2006 14:12:20 -0000
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With the figures you just gave, I would suggest you to connect the voltage from the RF sensor circuit straight to the base of an ordinary npn transistor, connect the negative terminal of your 10mA meter to the collector of this transistor, with the positive terminal of the meter going to a suitable positive supply (12v?). The emitter of the transistor goes to ground through a 220 ohm resistor. The voltage of the supply isn't critical.
To fine-tune the sensitivity you could make the 220ohm resistor adjustable.
As Dave says, there will be a slight 'knee' in the characteristic, so the
meter will read low if you run low power, and perhaps even not register at
all if the power is less than 1.75 watts, but if you will not normally
operate that low in power, you will probably never notice this.
Peter G3PLX

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