Arduino and measuring the output of a BC547C

[SniffTheGlove]

Hi,

I have been trying to create a simple project but I am stuck. I have tried asking for help in the Arduino Community Forums but all I get is responses of "Why do you want to do this" or "Don't use that, use this" or just cryptic responses that just confuse the hell out of me, so I am trying here for advice.

There several online tutorials you are shown how to create charge/voltage detectors using a couple of BC547 that drive an LED to show the charge etc being detected by an antenna. See the schematic here

So the level of brightness of the LED reflects the detected charge.

Now I would like to insert an arduino into the schematic so I am able to measure the strength of the detected charge so I can record the data and save to an SD Card.

Now this is where I can not get it to all work., so lets pretend I have an Arduino Uno and a breadboard (with the above schematic laid out).
The breadboard is feed by power from the Uno, along with the negative. Now where would I put the Analog (A5) Arduino connection to measure the detected charge or do I need to add something else.

I have tried to add A5 to the last BC547 base pin, and the emitter pin all without luck of getting a reading on A5 of the Uno

 

[Arouse1973]

Hello. How would I do this. I would get a phototransistor and measure the brightness of the LED using that. This solves two problems. One is it's non invasive so won't interfere with the circuit and it's operation. Two it will allow you to still use 9 volts for your supply voltage to the circuit and allow you to connect the phototransistor to the Arduino supply voltage.

Thanks
Adam

 

[SniffTheGlove]

Thanks Adam, I should of mentioned that in the final project I would be removing the LED from the circuit, which is why I am looking at measuring the output/charge/whatever

 

[Arouse1973]

For an indication that the level of charge is changing you will need to measure a point on the circuit that is also changing with a change in charge.

If you want to measure this using the A-D port of the Arduino then it might be a good idea to use a point that changes voltage or current over a reasonable range giving you a chance to measure it effectively. If you remove the LED but keep the resistor then you could use this signal connection from the collector.

However the voltage on the collector when not detecting charge is going to be 9 Volts. Ideally you need to reduce this level down to be compliant to the specs of the Arduino input voltage range which is most likely to be +/- 0.3 V above and below the supply rail respectively.

Some people use a current limiting resistor into the port hoping the over voltage diodes on the input ports clamp this voltage to prevent damage to the port pin. A lot of people do this however I don't so I am not condoning it, it's up to you. Other people would reduce this voltage to the correct level before it entered the port pin.

Also note that if you are able to measure a change in the level of charge it is going to be difficult to quantify the change without calibration using a know source and a calibrated EMF detector. You can hire this equipment from companies in the UK.

Thanks
Adam

 

[sumeryamaner]

I would;
- replace the LED and 220 ohm resistor with a 10 kohm resistor.
- connect a 1 kohm resistor to the collector of the rightmost BC547
- connect a capacitor (say 100 nF) from the other end of the 1 kohm resistor to ground
- connect the junction of the 1 kohm resistor with 100 nF capacitor to Arduino analog pin

To be on the safe side you can connect a 5.1V zener diode in paralel to the 100 nF capacitor.

Now you can perform analogRead() and the lower the reading the higher the sensor voltage.