I2C 16 relay board for use with RPi
There's a lot of switching to be done, for example on a model railroad (pun intended). In that environment we must be able to switch DCC block waves of 15 to 25 Volts / 1-2 Amps. The Raspberry Pi can not switch that kind of power by itself.
Purpose of this project is to create an add-on board with which we can switch DCC power (an other stuff) with 16 relays. These circuits will be controlled using a MCP23017, an I2C 16 channel I/O expander chip. This chip can be made to work at one of eight possible adresses on the I2C bus. This means that, when we indeed would use eight of these boards, we could control a maximum of 128 (!) relays via only the two wires on the I2C bus!
A solution to solve the insufficient electrical power of the Raspberry Pi is to make it control relays. This presents us with a challenge, because most relay coils take (much) more power than the Raspberry Pi can muster up from its 3,3 Volt outputs.
I chose to work with relays that will work on 5 Volts DC because my other available add-on boards also work on 5 Volts. The voltage of the logic chips (3,3 Volts) must be converted to the appropriate level of 5 Volts. Below you will find a number of solutions I found on the internet. Most of them use transistors as switches to do this conversion, controlled by a Raspberry Pi output pin.
Even with the transistor, the output of the logic chips has to be protected against to high of an output current. This is done by using a base-resistor that will be placed between the output of the logic chip and the base of the transistor. A value of a couple of K to 10K is recommended. Should this resistor not be present, there would be a short between the 3,3V output of the logic chip and GND (0V) through the transistor. This would end in the untimely demise of (the output of) the logic chip and most likely the transistor as well.
Another thing the circuit needs is a protection against the hysteresis effect. This is the effect that happens when you switch off power going through a coil. When you switch off the relay, its coil will produce a spike in voltage. To protect the circuit against this, we use an anti-parallel diode that shorts this voltage spike over the coil so that it does not reach the rest of the circuit.
A nice little extra would be to have an indicator LED to tell us when a relay is activated or not. We can place this LED, in series with a resistor, parallel over the relay coil. See also figure 1 below.
The idea you can see in this drawing, I found in this article at arduino-info.wikispaces. As you can see, a transistor is mentioned here,the BC847, but that's an SMD version. That's not so easy to solder in for 'old school' hobbyists like me. For the base resistor we see a value of 10K.
Another solution is the one I found at Stack Exchange, also uses a transistor. The picture below comes from this article at electronics.stackexchange. This one does not specify values for the components (resistor and diode).
Yet another, similar solution comes from susa.net. See the picture below, belonging to this article. Again, a transistor is used: a BC337. The resistor gets a value of 1 Kilo Ohm; the article text indicates that to be strict, if one wanted to stay below 3mA, this value should actually be 1,2K. The current then will be 2,75 mA.
At my local electronics shop, Okaphone, I found this relay, article number V23079-A1001-B301 (€6,95). With its 5 Volt coil-voltage, it can switch 2 Amps with 250 Volts AC or 220 Volt DC. See picture below:
At another shop (Farnell) the same relay could also be found, only there it would set us back a mere €2,72.
In the end I got - apart from the usual good advice - my relays for a good (lower) price.
Possibilities that were mentioned are:
- 'BC337', traditional
- 'BC547', same, but can only work max. 100mA
- 'BC847', SMD, no good.
Which diode we use actually doesn't matter that much, the standard diode 1N4007 will suffice.
I will opt for:
V23079-A1001-B301relay1N4007as TVS diode (€0,10)BC337-16(Old school, €0,20)- 10K base resistor (€0,01)
- 1K resistor protecting the current for the indicator LED (0,01)
- green indicator LEDs
L934LGD- current up to max 5mA (€0,20)
Furthermore I need:
MCP23017-E/SOI2C 16 bit I/O expander (€2,45)
The schematic based on the information so far.
In a later stage I discovered a far more elegant solution. There is a chip, the ULN2803A, that can drive eight relays using an internal darlington circuit and also contains an internal TVS diode. See the ULN2803A datasheet.
An (untested) PCB layout is available, I made it using Eagle.
I upgraded my schematic accordingly:
