One of the oddities of the RepRap community is the heated bed connector on the very popular RAMPS controller. It's undersized for modern beds and this is known for years, but nobody fixes it (which would trivial).

Anyways. One of the solutions to work around this situation is to use a SevenSwitch to do the hard work. Here we show how to connect this switch to a RAMPS and how to tweak the popular firmwares to work with the new switch instead of the old one.

The red oval marks the Servos connector.

First step is to find an appropriate signal pin on the RAMPS. “Signal” means, a pin which can be switched between 0 V and 5 V by the firmware, preferably using PWM. A RAMPS has a whole lot of extension connectors, we choose the Servos one. This connector features 4 PWM-able signal pins as well as convenient GND pins.

Some vendors solder in pins there already, others don't. If there are none, you have to do this yourself. Standard 0.1“ spaced pins are fine. Another option is to solder the signal wire directly into the board.

Some people tend to try connecting a SevenSwitch a different way. Let's elaborate on this.

Using D8 is a possible, but poor choice for several reasons:

• D8 is switched by a high current MOSFET, which is like using a truck for transporting a single cookie. There's no need for this MOSFET.
• Due being behind a MOSFET, signal on D8 is inverted. Without firmware modification, commanding the heater to be off turns it actually on and vice versa.
• There are better uses for this output, e.g. for driving a fan, lighting, heaters, whatever draws less than 10 amps and has no coil inside.
• If the RAMPS is operated with 24 volts, it'll destroy the SevenSwitch's signal pin, which can withstand 20 V, only.