So far I’ve attached devices to my Raspberry Pi or Arduino Uno with ratings less than 100 mA. These devices were connected to the output pins through a transistor or optocoupler. However, for heavy load currents a relay must be used. As a noob, it is somewhat difficult to chose parts with the right specs for driving a relay — so here are some hints.

Driver circuit

The driver circuit should be arranged as on the scheme below. The digital output pin of the microcontroller (J1) is connected through a resistor (R1) to the base of transistor Q2. The collector of Q2 connects to +5V and the emitter to the relay coil. A free-wheeling diode is inserted across the coil.

To build an Arduino/ATtiny relay shield, I used a 5 Volt HJR-4102-L mount relay, with a coil resistance of 125 Ω. Thus, the collector current (I_c) of the transistor should be at least 2 x 5 / 125 = 80 mA. The popular BC337-40 transistor can handle up to 800 mA and therefore has the right specs to drive this relay.

Calculating the size of the base resistor, R₁

The DC gain (h_FE) of BC337-40 is about 250-600. Since there is no h_FE(SAT) given in it’s datasheet, we calculate with an estimated h_FE under saturation of 250 / 3 = 80. In order to reach a coil current of ≤ 80 mA, the transistor base current (I_B) must be at 1 mA (I_B = I_c / h_FE). According to it’s datasheet, the transistor base of BC337-40 can deal with a current of 100 mA. An Arduino UNO pin can handle up to 40 mA. Thus, we’re not exceeding the limits.

The Arduino UNO microcontroller can deliver up to 5 Volt (minus 10 {c7f7cb1468c0d02af358b3ce02b96b7aadc0ce32ccb53258bc8958c0e25c05c4} tolerance = 4.5 Volt at 1 mA). With a voltage drop of 0.7 V between base and emitter, we can now calculate the size of the base resistor as: R_b = (4.5 – 1.2) / 1 = 3.3 kΩ.

Arduino/ATtiny relay shield

I placed the relay on a small piece of stripboard together with the free-wheeling diode and a 6 x male pin header to access both relay coil and switch pins.