Advent Electronica #3

By Jennifer Sanders on December 17, 2015

Advent’s back, back again.

Day 11 and time for another 1 kΩ resistor, this time so we can use the red LED as a photodiode, or light sensor. When the red LED’s cathode is connected to the battery, it will conduct a tiny bit of current — on the order on nanoAmps — from any bright light source. Having two transistors — known as a Darlington pair — allows the current to be amplified and will power the green LED.

A dimmer was the order of day 12 with the arrival of a 100 nF disc capacitor, where the intensity of the green LED can be adjusted using two finger touch “buttons” (stripped pieces of wire).

Things started to heat up on day 13 and 14 where a 1 MΩ resistor allowed us to create a temperature sensor. Touching the charged capacitor with your finger, heats up the capacitor and dims the LED. Infrared temperature sensors work in a similar way, albeit with more sophisticated amplification. Another 100 nF capacitor on day 14 to pimp my circuit to a twin temperature sensor! Depending on which capacitor you touch, you can dim or illuminate the LED, or even create an alternating light show.

Day 15 revealed more wire for creating an electrical field sensor using an antenna. Like above, it uses the Darlington pair to detect my contact with an insulating surface, such as our lovely office carpet. All without me having to touch the wire, just placing my hand close to it. The process – electrostatic induction – results in a flashing LED. You can even use the circuit to detect the electric field in power cables e.g. your mobile phone charger.

At day 16, we now had a trio of transistors. Now, we can detect the electrical field of people passing by, up to six feet away! This is due to the amount of static charge that builds up on and around our bodies.

We’d not seen the end of temperature sensors and Day 17 brought us another resistor; this time a 4.7 kΩ resistor at 25°C where its resistance is inversely proportional to temperature. These are known as negative temperature coefficient (NTC) resistors which don’t require such drastic temperature changes as normal resistors to affect their resistance. Using this one in our circuit means the LED only lights up if the temperature drops below 25°C. In this way, we essentially created an LED thermometer! Rather different to the old mercury thermometer.