This Instructable shows you how to make a small signal detector from old recycled components.

A signal from the sensor is usually amplified before fed into a microprocessor or microcontroller analogue to digital converter inputs. An alternative is a circuit shown in this Instructable, a LED detector with no processing. However, this circuit does not include the amplifier. Only the detector. When no signal is applied, the LED is turned OFF. When a small amplitude sine wave is applied then the LED turns ON.

Supplies

parts: LED - 2, general purpose NPN transistors - 2 maximum, wires, matrix board, 470 uF or 100 uF bipolar capacitor, 5.6 kohm resistor, 100-ohm resistor or two 220 ohm resistors connected in parallel, 100 kohm resistor.

optional parts: solder, metal wire (1 mm), 10 uF capacitor.

tools: pliers, wire stripper

optional tools: soldering iron, multimeter

Step 1: Design and Simulations

I specified the Cin, 470 uF bipolar capacitor because the input signal can be of very low frequency. However, I implemented the circuit with just 100 uF bipolar capacitor. You can replace Cin with a short circuit to increase the circuit gain if the minimum input signal voltage falls below 0.7 V during transient waves oscillation loops.

I used old PSpice software to reduce drawing time. LED is modelled with three general-purpose diodes.

Rled = (Vs - Vled) / IledMax = (3 V - 2 V) / 10 mA = 100 ohms

Rin is very important for making sure that the transistor is OFF when no signal is applied. Rin discharges the Cin capacitor when the transistors are in the OFF state. This happens (transistors in OFF state) when the base-emitter voltages are below 0.7 volts.

Simulations show 8 mA current across the LED.

Step 2: Build the Circuit

The blue and the orange wires are the inputs. The blue wire is used for signals that fall below 0.7 volts. Thus eliminating the need for input capacitor Cin.

I attached a 100 uF capacitor across the power supply. You might want to use a 10 uF capacitor because long charging times caused by a high Farad value power supply filtering capacitors can drain high current from batteries and they can become hot.

I had to use two transistors because those are old Soviet transistors (that I received in the mail) and cannot handle high-power signals. Those transistors could be older than me :-)

Step 3: Test the Circuit

The blue and the orange wire are the inputs.

Do not connect the orange and the blue wire together. You will be shorting the input capacitor. Those are two separate inputs. At least one of those inputs should be left disconnected.

Do not forget, this circuit cannot be connected directly to the sensor due to low gain. It needs an amplifier in between the sensor output and this circuit input.