This is a simple controller I designed for operating the linear actuator used in a piece of electromechanical furniture. I use microcontrollers in so many of my projects, but on this occasion I thought I'd use 4000 series logic and four 555 timers (two 556 timers) as these can all operate at 12V. This eliminates the need for a voltage regulator, which would be required if a microcontroller were to be used.
The circuit consists of two 556 dual timers, a CN4011 NAND gate, a CD4066 quad-bilateral-switch, and an H-bridge arranged from N-channel and P-channel MOSFETs.
The controls provide the following functionality
The power switch completely disconnects the battery, so no quiescent current is drawn from the battery
The illuminated red button, while (held) pressed powers the actuator, driving it towards its closed position. Pressing this button starts the delay-before-opening timer, and flashes to indicate that it is operating.
The illuminated green button, upon a brief momentary press, will illuminate and keep the actuator powered until it has reached its fully extended (open) position. A momentary press of this button cancels the delay-before-opening timer, started by pressing the red button.
The rotary switch selects the delay-before-opening duration, providing six positions ranging from twenty minutes to four hours.
There is also a screw-terminal connection (JAM SW) for an optional obstruction-detecting microswitch. When this connection is shorted by the microswitch, both the red and green buttons are illuminated to indicate they have been temporarily disabled, and the actuator is powered until it reaches its fully extended position.
How it all works
The first 555 timer is used in its monostable configuration to drive the actuator for a duration adequate to extend it fully from its closed position. Triggering of this timer resets the second.
The second timer is used to provide the 'delay before opening' functionality. Once it has been triggered by pressing the red button its output will remain in a high state according to the position / resistance selected by the rotary switch. The output is connected to a NAND gate configured as an inverter, so when the timer's output goes low the output of the inverter goes high and charges the 1uf capacitor (C14) which in conjunction with the 10k resistor (R7) creates a short high pulse that briefly enables the forth analogue switch of the CD4066. This analogue switch starts the first timer by momentarily connecting its trigger pin to ground.
The third timer is responsible for flashing the red button's LED using an a-stable configuration. Its reset pin is connected to the output of the second timer, and as this pin is active-low when it's in a high state the LED flashes, and when low the LED is off.
The forth timer is triggered by the optional obstruction switch. When its output goes high it enables the forth analogue switch of the CD4066 which triggers the first timer and resets the second. But it doesn't rely on the first timer to operate the actuator, instead it disconnects the first timer's output and the red button from the h-bridge by disabling analogue switches one and two, then enables switch three which enables the right-side of the h-bridge. This provides the effect of disabling the buttons, both of which are illuminated by the output of the forth timer via the arrangement of diodes D6 to D9.
Each of the six positions of the rotary switch increases the resistance by switching to a higher value resistor. As the timing is provided by an analogue RC circuit these times are only approximate. Listed below are the resistances and times I measured for each switch position, and the digital equivalent I would have chosen if this were a microcontroller based system.
POS | RESISTANCE | MEASURED TIME | EQUIVALENT 1 | 985k | 22m (0h 22m 06s) | 20 Minutes 2 | 2.24M | 47m (0h 47m 29s) | 40 Minutes 3 | 3.36M | 1h 20m (1h 19m 47s) | 80 Minutes 4 | 4.70M | 1h 56m (1h 56m 43s) | 120 Minutes 5 | 6.90M | 2h 48m (2h 48m 32s) | 180 Minutes 6 | 9.99M | 4h 06m (4h 06m 02s) | 240 Minutes
Die cast aluminium control box housing PCB and controls
The controls are mounted in a robust die cast aluminium enclosure. I used a small milling machine to drill and mill the cut-outs for the buttons and power switch. The PCB is mounted on four adhesive backed stand-offs, securing it to the enclosure.
Milling and drilling the enclosure for buttons and switches
Enclosure assembled and ready to be mounted
Linear actuator with 200mm of extension
