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Curtain Control Circuit. Part 1

Andy Collinson


This hybrid circuit uses a mixture of transistors, an IC and a relay and is used to automatically open or close a pair of curtains. Using switch S3 also allows manual control, allowing for curtains to be left only partially open or closed. The circuit controls a motor which is attached to a simple pulley mechanism, to move the curtains. I first started this circuit over 20 years ago and apart from now using metal gears, very little has changed.

Curtain Control Circuit
Click to enlarge


Automatic Operation

The circuit can be broken down into three main parts:

  • a bistable latch,
  • a timer,
  • a reversing circuit.

Toggle switch S3 determines manual or automatic mode. The circuit as shown above is drawn in the automatic position and operation is as follows. The bistable is built around Q1 and Q2 and associated circuitry and controls relay A/2. S1 is used to open the curtains and S2 to close the curtains. At power on, a brief positive pulse is applied to the base of Q2 via C2. Q2 will be on, and activate relay A/2.

The network of C3 and R4 form a low current holding circuit for the relay. Relay A/2 is a 12V relay with a 500 ohm coil. It requires slightly less current to keep a relay energized than it does to operate it. Once the relay has operated, the current through the coil is reduced by R4, saving power consumption. When Q2 is off, C3 will be discharged, but when Q2 becomes active (either at switch on or by pressing S1) capacitor C3 will charge very quickly via the relay coil. The initial charging current is sufficient to energize the relay and current flow through R4 sufficient to keep it energized.

Q1 bias is applied via R3 which is tied to Q2 collector. As Q2 is on, the collector voltage will be low, close to 0v and therefore Q1 and LED L1 will be off. As Q1 is off, its collector voltage will be high, and Q2 bias voltage is applied via the chain L1, R1 and R2. The curtains should already be fully open.

If now S2 is pressed, the base voltage of Q2 will become 0 and Q2 will switch off. In switching off, its collector voltage will rise to the supply voltage and Q1 will now be forward biased via the relay coil A/2, R4 and R3. LED L1 will now be lit, relay A/2 will be de-energized and as Q1 collector will be low, Q2 will be off and the circuit latched in this condition.

At the same time as S2 is pressed, the trigger input of IC1, a 555 timer (normally held high via R7 will be taken low. A timing sequence now commences. Duration is controlled by preset P1 and C6 and the timing is adjustable between about 1 and 12 seconds. This delay is adjusted so that the motor will run for sufficient time to fully open or close the curtains. The output of the 555 turns on Q3, fed via R8 which now applies power to the motor via relay contacts A1 and A2.

At any time the motor is in operation, and for any direction, LED L2 will always be lit. Contacts A1 and A2 reverse the polarity of the voltage appearing at the motor terminals, for more help on relays and switch contacts, visit this page in my practical section. A running motor generates a back EMF and D4 and D5 prevent this voltage from destroying the IC and transistors.

Manual Operation

If the toggle switch S3 is changed to manual mode, operation is slightly different as outlined below. The bistable latch formed around S1, S2, Q1, Q2 and associated circuitry operates the same way as in automatic mode.

S1 and S2 set or unset the bistable circuit which control relay A/2 and determine the direction of the motor. In addition, as long as either S1 or S2 is held pressed, a bias current will flow through either D1 or D2 and R6 into the base of PNP transistor Q4. This small base current results in a larger collector current flowing via R9 into the base of Q3. The BD139 will now be fully switched on and drives the motor as long as either S1 or S2 is pressed. Hence it is now possible to partially open or partially close the curtains. If you prefer a manual control then the following simpler electrical circuit is available.

Curtain Control Circuit

The close switch applies power to the motor via the relay contacts. The 1N4001 diode prevents the relay from operating. When the open switch is pressed, the relay is operated and power is again applied to the motor, though this time the contacts have changed and the motor will turn in the opposite direction.

To be continued

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