Hello everyone, I have a gieger tube circuit which has the following design:
Where
R1 = 220KΩ
R2 = 2.2MΩ
C1 = 2200pf Rated for 1000V
HV = 400V
Rpd = ? (internal pull down resistor of my oscilloscope)
I have to read the signal as a pulsed square wave into a PIC micro (so there is a resistor tying the signal to ground (pull down) to read the voltage across). I have two cases I have to sort out:
1.) Protect the micro from really high voltages
2.) Make signal square like and not dirty
My assumptions, GM tube can be modelled as a switch which closes only when radiation hits it, other than that it is just a open circuit. Signal is measured across Rpd (pull down resistor of Micro)
1. When I first turn on the HV source, C1 charges up through R1 and the Pull down resistor of the micro. This is a quick boost in current to get the cap to 400v, the same as the HV source. So I see it as a positive spike on my osciloscope. I need some way to 'not' see this as the micro is not rated for that high of a signal.
2. When the GM tube does fire, it creates a voltage divider between R1 and R2 which drops the voltage over R2 to about 390v. Now the Cap has to drain off 10v to match this change and does so by running current through Rpd in the opposite direction as when the cap was initially charged in 1. So you see a negative voltage drop across Rpd. Next the GM tube stops conducting and we get the voltage back to 400 so the cap must recharge, and does so by a positive voltage.
I need a way to clean this signal to a positive square wave triggered only when the voltage drops anywhere lower than -1v say. I'm going to attemp to draw my signal here ha.
I hope thats readible ha, but anyway, does anyone have any suggestions for signal processing, links? Books? I just need knowlegde I was not really trained in circuit design so much as physical processes (Physicist). I'm leaning toward using diodes some how and a separate 5v power supply, but am a little lost. (2-5v is about the size I want to input into the micro)
Anything you could post would probably help me.
Rich (BB code):
HighVoltage ----- R1 -------o------ R2 ------- GMTube --- GND
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C1
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Signal -- Rpd --- GND
R1 = 220KΩ
R2 = 2.2MΩ
C1 = 2200pf Rated for 1000V
HV = 400V
Rpd = ? (internal pull down resistor of my oscilloscope)
I have to read the signal as a pulsed square wave into a PIC micro (so there is a resistor tying the signal to ground (pull down) to read the voltage across). I have two cases I have to sort out:
1.) Protect the micro from really high voltages
2.) Make signal square like and not dirty
My assumptions, GM tube can be modelled as a switch which closes only when radiation hits it, other than that it is just a open circuit. Signal is measured across Rpd (pull down resistor of Micro)
1. When I first turn on the HV source, C1 charges up through R1 and the Pull down resistor of the micro. This is a quick boost in current to get the cap to 400v, the same as the HV source. So I see it as a positive spike on my osciloscope. I need some way to 'not' see this as the micro is not rated for that high of a signal.
2. When the GM tube does fire, it creates a voltage divider between R1 and R2 which drops the voltage over R2 to about 390v. Now the Cap has to drain off 10v to match this change and does so by running current through Rpd in the opposite direction as when the cap was initially charged in 1. So you see a negative voltage drop across Rpd. Next the GM tube stops conducting and we get the voltage back to 400 so the cap must recharge, and does so by a positive voltage.
I need a way to clean this signal to a positive square wave triggered only when the voltage drops anywhere lower than -1v say. I'm going to attemp to draw my signal here ha.
Rich (BB code):
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0v-------------------------------
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Anything you could post would probably help me.