Camper: Lock the drawer in the kitchen when the engine is running
There are 1 or 2 comercially availble solution on the market, but I wanted to diy it. The project has a lot of mechanical parts that I 3D printed. The metal is of the shelf from a DIY store.
The electronics consist of moduls from China.
1x CPU ATTINY88 (15 x 2 Pin Development Board)
1x Double Optokopler PC817 board (In 3,6-24V 5mA / Out 3,6-30V 50mA max)
1x Double Relayboard 12V
All electronics are rated well above +12V and therefore no extra power supply is needed. The housing is 3D printed as well.
The tricky part was to get the ATTINY88 into sleep mode, so it doesn't use much energy. At the end I swapped the 78L05 on the board against BA33BC0 (180° twisted to match the pins) and reduced the voltage to 3,3V. All Power_on Leds had been removed. In sleep mode the electronics draws 2mA on 12V. Not perfect but ok. The camper has Solar Cells on the roof and is re-filling this small amount easily.
The CPU sleeps until the D+ signal (+12V) of the engine wakes it up. Then it cycles to the close position. If D+ is 0V, the CPU wakes up again and releases the drawers.
The little servomotor is from a Car Junkjard for about 5€. It was taken from an Audi Trunk opening mechanism. Many car vendors use identical servomotors. Some have a spring to reset themselfs. The spring can be easily removed. We found one without a spring. So changing polarity moves it from 1 position to the other and the engine and gears are strong enough to hold that position. The servo needs ~1 second to change position.
A wire with the D+ signal as well as +12V and Ground is needed. Luckily a connector with allways on power was already near that position.
The pictures show how the mechanics is installed. In case the engine is running or the electronics breaks, we added a possibility to move the mechanics with a little lever by hand, from the other side of the wall. The mechanic has to be flat enough, so it fits sideways between the wall and the drawer side panel.
The little wingshaped "lock" needs to fit snug with the bold. A bit tricky to find the perfect position :-)
This is a DIY project and nothing commercial. Depending on your drawer, the mechanics needs adapting. If by any chance you own a Dethleffs Just Go Camper, you may be able to copy it 1:1.
In any case it is all at your own risk !!!