Using a UVtron with and without an oscilloscope

Other Hardware General Electronics
1

I am working with an arduino and a UVTron sensor. I want to put this setup on a mobile platform and have it set off an alarm when the sensor picks up UV.

The problem I am having is when I have an oscilloscope in the loop to monitor the signal sent by the sensor the circuit works. When I try to condense the circuit without the scope in the loop no signal is sent to the arduino.

I've attached to diagrams of how I have it hooked up. I haven't changed anything except the wiring in the diagram when I pull the scope out of the loop. After I took the scope out of the loop I also tried a separate program that does a pulseIn() command to read the input pin and print what is received but I get nothing.

Simple alarm code

void setup() {
  pinMode(24,INPUT);
  pinMode(25,OUTPUT);
}

void loop() {
if (digitalRead(24) == HIGH)
  {
    digitalWrite(25,HIGH);
  }
else 
  {
    digitalWrite(25,LOW);
  }
}

Debug code

int pin = 7;
unsigned long duration;

void setup()
{
  Serial.begin(9600);
  pinMode(24, INPUT);
}

void loop()
{
  duration = pulseIn(pin, HIGH);
  Serial.println(duration);

}

UV sensor with scope.png
UV sensor with scope.png682×878 5.99 KB

UV sensor without scope.png
UV sensor without scope.png682×878 5.02 KB

2

Maybe try:

pinMode(24,INPUT);
change to
pinMode(24,INPUT_PULLUP);

3

Weird!!!

Are you using an actual bench-top oscilloscope or something that attaches to the computer?

A regular oscilloscope usually has a very-high impedance so it should have no effect on the circuit. (There is also some capacitance which can have an effect on high-frequency or fast-switching circuits, but it should have no effect on low-impedance DC circuits.)

Do you have a link to the specs on the UVtron sensor?

What's the value of the pull-up resistor on DIN? (Maybe the sensor want's a pull-down?)

When I try to condense the circuit without the scope in the loop no signal is sent to the arduino.

Is it stuck high, or stuck low?

4

I apologize for wasting your guys time. I looked at the datasheet with a lab mate and we found another method to wire the circuit that works without the oscilloscope in the equation. Thanks.

5

DVDdoug:
A regular oscilloscope usually has a very-high impedance so it should have no effect on the circuit.

No, not really the case, standard scope input is fairly low impedance at 10MHz because of the input capacitance,
which is why the x10 probe is commonly used (to reduce the capacitance by a factor of 10).

Scope might be something like 50 to 100pF with a dumb coax probe lead, perhaps 30pF actual
input capacitance at the front panel and with a decent x10 probe more like 5pF.

At 10MHz that's 160--320 ohms, 530 ohms and 3k1 respectively... Not that high!