A motor, a switch, a battery, simple and cheap. What could be easier ? That was my thoughts... until reality came knocking.
Simple and cheap meant using only passive components. No stepper motor, no smart switch detection logic controlling a power stage, no electronics for reversing motor direction.
The only active electronics will be the DC-DC bulk converter that provides an adjustable voltage output so the motor's RPM can be set as needed, and independently of the state of charge of the battery.
Since the motor power is 60W, the max sustained current should be 5A @ 12V. For safety reasons, and to account for possibly crappy Amazon-sourced parts, all the components will be rated for at least 10A @ 12V DC.
The DC-DC converter used is a no name basic one: ANMBEST DC 6-40V to 1.2-36V 20A 300W Adjustable Buck Converter
Controlling the direction of the brushed motor is done by reversing the motor's + and - poles. That can be accomplished with a DPDT switch (dual pole dual throw).
For extra safety the DPDT is of the temporary type
that requires the user to maintain pressure while operating. Should
anything happen, the user just has to lift the finger off the switch.
The Blue Sea Systems' 7484 (ON)-OFF-(ON) temporary switch is
conveniently rated for 10A at 12V DC.
Upper and Lower limit switches stop the motion of the bed. They also need to be rated for 10A @ 12V DC. Hopefully, those switches from Amazon, rated at 10A @ 30V DC, will honor their specs ?
However, once a switch opens it cuts the power to the motor. So an alternate path with a diode must be provided to allow reversing direction. Like 2-wire electrical actuators do.
Which leads us to this exciting electrical diagram:
Note the Flyback diode to bypass the negative voltage spike generated by the motor when the current gets interrupted by a switch. That protects the the switch contacts and the DC-DC converter. Says me after 💥blowing up💥 the initial 10A DC-DC converter...
Testing
showed a notably higher speed when lowering the bed, because... gravity
! For safety reasons, for instance if the cat is still sleeping on the
countertop, the downward speed needs to be reduced.
So diodes inserted on the 'going-down' electrical path lower that
voltage.
Worked like a charm, no need for a 2nd DC-DC converter bridged
in. And we are saving cats 😸
Bed in top position, ceiling locks engaged (driving mode):
The carriages / chains / gears are so strong that, statically, they can easily handle
200-300kg (450-650lbs) add'l weight on the bed. And once the bed is down and resting
on the countertop, and the extension resting on ribs deploying from
under the bench's lid, 4 elephants could easily square dance on the bed !
Only good nights to be had now !
That was somewhat easy peasy. Well, except for the 3 fuses blown on this 'simple' project... But hush, nobody needs to know that I'm a klutz 😅
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>>> next post: A Sink or a Shower or a Toilet... ? Yes !
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