Circuit : Andy Collinson
Email: anc@mitedu.freeserve.co.uk
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Description
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.
Automatic Operation
The circuit can be broken down into three main parts; a bistable latch, a timer and 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.
Read More Source:http://www.zen22142.zen.co.uk/Circuits/Interface/ccc.htm
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This entry was posted on Saturday, October 6th, 2007 at 11:19 am and is filed under Electronic Control, motor speed controller.
