3
\$\begingroup\$

I have an i-Snail-VC-50 sensor with an output of 0-5V. I'd like to connect this output to the base of a BC547 transistor, which powers an LED with an external 5V power source. What I want to achieve, is that the LED lights up as soon as the sensor output voltage rises above 0V (most of the time, the output will be either 0V or within the 0-1V range).

How could I achieve this, making sure that no components get damaged if the sensors output is at its maximum (6.5V)?

I know transistors are current driven, so perhaps a voltage driven mosfet (I have some IRFZ48N's at my disposal) would be better suited for this task?

\$\endgroup\$
3
\$\begingroup\$

You can do this with a BC547 transistor, but you'll need an op-amp or comparator to amplify the sensor signal first. The transistor won't turn on until you raise the base 0.6 V above the emitter, so for your 0-1 V range, it would be off most of the time. Even above the 0.6 V threshold, the current drive capability of the sensor is very low, so the LED would only weakly turn on.

Once you amplify the sensor signal, you can use the transistor as a digital switch. On this linked page, see the diagram under example #2, labeled Digital Logic Transistor Switch.

\$\endgroup\$
  • 1
    \$\begingroup\$ I already intented to use it as a digital switch, as it would only have an 'on' and 'off' state. I'll have to get to know Mr. Op-amp then :-) Thanks! \$\endgroup\$ – Vincent Van Den Berghe Nov 1 '10 at 19:18
  • 2
    \$\begingroup\$ Getting to know Mr. Comparator might serve you better, given that you want a digital output. His friends call him by the nickname LM339. \$\endgroup\$ – pingswept Nov 1 '10 at 19:49
3
\$\begingroup\$

pingswept is right about the amplification, but he cuts a few corners. First, if you amplify a 0V-5V signal, the amplified signal range still starts at 0V! We'll need to fix that. The i-Snail is self-powered, which means it needs a minimum current to be able to output a voltage. So at no current the voltage will be effectively 0V, but it's possible that the lowest effective output voltage is, say, 5mV. The datasheet doesn't say. For the given sensor that would mean a 50mA primary current.

To amplify this low kind of level you need a rail-to-rail input-output opamp. A 100x non-inverting amplifier would amplify the 5mV input to 500mV. That's still too low to drive the transistor, but we can use a comparator to compare this with a threshold of, say, 100mV. Many comparators have open collector outputs, but those can often sink less than 20mA, so you'll want to drive the NPN with the comparator after all.

Alternatively you can skip the amplifier and directly input the i-Snail's signal to the comparator, and set the threshold to 5mV. Keep in mind however that this will be near the comparator's offset voltage, so you may have to trim that. It also depends on the minimum voltage the i-Snail delivers or the minimum level you want to detect. In the given example of 50mA that would be 6W at 120V AC.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.