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PROXIMITY DETECTOR

(with on-delay feature)

Low power unit operates on 3AA cells which last for at least two years. Plates mounted under wooden stairs (top and bottom) turn on stairwell lights for approximately 35 seconds.



This project was generated to lighten a dark stairway when a person sets foot on the first step. Plates mounted under top and bottom stairs turn lights on when person starts up or down the stairs.

A low frequency oscillator, large resistance values, a latching relay, and use of CMOS circuitry are responsible for its low power consumption resulting in excellent battery life from 3 - AA cells.

The low frequency oscillator is designed using the CD4007, A1. The pulse is shaped and sharpened by A4, pins 1,2, and 3.

The circuit around A3, CD4013 - a "D" flip flop, is actually a very sensitive capacitance change detector. It is automatically kept in balance. Any imbalance due to someone nearing the sensing wire will be detected.

How it works:

One section of the CD4013, "D" flip flop normally has a balanced input going to the direct set and direct reset inputs (ST2 and RST2). C1 and C2 are discharged by the common negative low of the clock signal from A4-3. Depending upon which input (ST2 or RST2) "releases" control of the flip flop last -- determines the state Q2 will take. The state of Q2 is latched in during the positive going rise of the A4-3 oscillator clock signal. This is output as a feedback signal at NOT Q1.

In actuality, the signal at NOT Q1 is constantly alternating between high and low to give proper feedback signal to the servo loop filter formed by the 30 megohm resistor, and C3 (200 ufd. low leakage electrolytic capacitors). This servo loop is completed by means of A2 pins 3,4, and 5 which constantly adjusts the resistance leg of C2, keeping a match to the time constant -- that of the sensing wire, 47K resistor, and C1.

A5 (CD4020) is a 14-stage counter. A5 is constantly being reset if the circuit is in balance. Should, however, an object approach the sensing wire, A5 starts counting. When a given count of 32 is reached, the delay bistable is set to on.

At the same time, a reset is given to the other CD4020 (A6) counter. When A6 reaches the count on the 12th division, the delay bistable is reset to off. By selecting other division pins, other delays can be achieved.

I used aluminum foil sheets (about 10" X 24"), stapled to the underside of the top and bottom steps of a 13-step staircase. These served as good sensing plates for wooden stairs with double padding and rug. The two sheets were connected to a common line which ran to the sensing wire antenna. An earth ground from a pipe was used for the circuit's ground return.

One other note. A small reed relay with a small magnet mounted on the top converts this standard small I.C. type relay into a latching bistable unit. Careful positioning of the magnet on the relay is required to achieve these results. Once cemented in place, the relay will turn on with a short positive going pulse and turn off with a negative going pulse. This technique was used to maintain the extremely low power drain of the circuit. Typically, the circuit during idle draws only about 1.5 microamps.

IMPORTANT NOTE: This unit requires several hours to "settle" after batteries are installed. This is because until capacitor C3, the 200 uf. capacitor, reaches approximately half supply voltage, the circuit cannot function.

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Page Modified January 12, 2001