Any components to determine the charges?

[oem7110]

I would like to make an electronic electroscopes based on following concept, but its design cannot determine whether the charges are positive or negative.
Does anyone have any suggestions what kind of components I can add into this device to determine the charges?
Thanks in advance for any suggestions

http://amasci.com/emotor/chargdet.html

 

[vk6kro]

That circuit will draw more current if the voltage is positive and less if it is negative, so you can easily tell which is which.

I built something similar years ago, with a meter instead of the LED, and it worked, but the FET tended to get destroyed fairly easily.

I suppose you could hunt for FETs with built-in static protection.

 

[oem7110]

That circuit will draw more current if the voltage is positive and less if it is negative, so you can easily tell which is which.

I built something similar years ago, with a meter instead of the LED, and it worked, but the FET tended to get destroyed fairly easily.

I suppose you could hunt for FETs with built-in static protection.

Do you have any suggested model for FETs with built-in static protection?
The store only provide service within US, and I cannot find this model in my local area. MPF102 Field Effect Transistor (FET)

Do you have any suggestions?
Thanks you very much for any suggestions

 

[oem7110]

Does anyone have any suggestions on any substituted components I can use into this device to determine the charges?
Thanks everyone very much for any suggestions