Small Circuits Revival (27): Simple Ion Meter
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idea: Elex-team
Simple Ion Meter
It has been asserted for some time that negative ions can have a positive effect on the well-being and mood of people. Admittedly, that positive effect of negative ions is mostly claimed by manufacturers of such so-called ionisers; reliable scientific studies have not been able to substantiate these health claims. Nevertheless: in these times of lockdown as a consequence of ‘that virus’ it is important that the indoor climate is as healthy as possible. And with the circuit that we present here, you can easily check whether the air around you has sufficient negative ions.
The schematic shows that not a lot is required to detect the presence of negative ions. The ions are ‘caught’ (or collected, if you prefer) with a small metal plate (the shaded square in the schematic). When there are positive (and therefore ‘harmful’) ions, then nothing happens. However, when the ‘good’ negative ions are in the majority, then the base of transistor T1 will become negative relative to the emitter: the transistor will start to conduct.
![](https://cdn.xingosoftware.com/elektor/images/fetch/dpr_1,w_600,h_440,c_fit/https%3A%2F%2Fwww.elektormagazine.com%2Fassets%2Fupload%2Fimages%2F42%2F20200510175517_22399-01.jpg)
The same is true for T2 and obviously also for T3. These three transistors form a kind of triple-stage super-Darlington – the total gain then equals the product of the three individual gains; plenty of gain therefore. It should be clear that even the slightest negative charge on the small metal plate will cause T3 to start conducting; the current through this transistor is made visible by moving coil meter M1. You can not only see whether there are negative ions at all, but the deflection of the meter also gives a rough indication as to how many there are.
The construction on a small piece of prototyping board is not critical; however, the wire connecting the metal plate and resistor R1 should be as short as possible. The adjustment of the circuit with trimpot P1 depends entirely on the local surroundings. You can test its operation in the bathroom: when you turn on the shower many negative ions are released. That could, incidentally, explain very well why a nice shower is so relaxing...
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