3V CAPACITANCE PROXIMITY SWITCH

This circuit was designed to provide a touch activated switch function without an external power supply. It draws so little power that a single 3v battery will operate the circuit for many years.  It is discussed in more detail in the section on Capacitance Proximity Switch Technology.
Click on Drawing Below to view PDF version of Schematic

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1.5V TOUCH ACTIVATED SWITCH

A single 1.5v silver oxide button cell powers this complete touch activated switch circuit for 5 years.  It features both a normally open and a normally closed set of solid state switch thermals.  It also has an adjustable sensitivity, which can be set for a touch capacitance change as small as 1 picofarad.

Click on Drawing Below to view PDF version of Schematic

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Touch sensor

AT90S2313

This is a simple circuit which can detect when you touch a sensor connected to one of the sensor inputs. It can be used to add a touch switch to your computer for example. It uses an AVR micro controller, the AT90S2313. This is overkill. I can add a lot more sensors than the two sensors I have now, but I had a 2313 to spare and I didn't need more than two sensor :). Anyway, the basic idea is really simple. Make one pin output and another input. Connect a resistor between these pins. The resistor together with the human capacitance (about 100 pF) forms an RC network. The AVR set the output to low and then make a transition from 0V to +5 V. 5 µs after this switch, the logic level at the input pin is sampled. If someone is touching the probe connected to the input pin, the capacitor (=human) will not be fully charged, and the input will be a digital 0 and vice versa.

Schematic and board layout

This is the schematic. The potentiometers are there to adjust the sensitivity. 0.5 MOhm is usually a good value.

This is the board layout (made in EAGLE)

Component list:

Name	Value	Package
R1	4.7 kOhm	1206
R2	1 MOhm
R3	1 MOhm
R4	3.3 kOhm	1206
R5	3.3 kOhm	1206
Q1	BC547	TO-92
Q2	BC547	TO-92
IC1	AT90S2313	DIL20
XC1	8 MHz Murata ceramic resonator
ISP_CON	AVR isp connector
SENSOR1	1 x 2 pin header, sensor 1 connector
SENSOR2	1 x 2 pin header, sensor 2 connector
SWITCH1	1 x 2 pin header, output for sensor 1
SWITCH2	1 x 2 pin header, output for sensor 2
SUPPLY	1 x 2 pin header, +5 V and GND

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4 Digit Keypad Switch

Description

This is a universal version of the Four-Digit Alarm Keypad. I've modified the design of the output section - to free up the relay contacts. This allows the circuit to operate as a general-purpose switch. I used a SPCO/SPDT relay - but you can use a multi-pole relay if it suits your application.

Do not use the "on-board" relay to switch mains voltage. The board's layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you want to switch mains voltage - mount a suitably rated relay somewhere safe - Away From The Board.

Schematic Diagram:

Notes:

The relay is energized by pressing a single key. Choose the key you want to use - and connect it to terminal "E". Choose the four keys you want to use to de-energize the relay - and connect them to "A B C & D". Wire the common to R1 and all the remaining keys to "F".

The Circuit is easy to use. When you press "E" - current through D2 & R9 turns Q6 on - and energizes the relay. The two transistors - Q5 & Q6 - form a "Complementary Latch". So - when you release the key - the relay will remain energized.

To de-energize the relay - you need to press keys "A B C & D" in the right order. When you do so - pin 10 of the IC goes high - and it turns Q4 on through R8. Q4 connects the base of Q6 to ground. This unlatches the complementary pair - and the relay drops out.

Any keys not wired to "A B C D & E" are connected to the base of Q3 by R7. Whenever one of these "Wrong" keys is pressed - Q3 takes pin 1 low and the code entry sequence fails. If "C" or "D" is pressed out of sequence - Q1 or Q2 will also take pin 1 low - with the same result. If you make a mistake while entering the code - simply start again.

The Keypad must be the kind with a common terminal and a separate connection for each key. On a 12-key pad - look for 13 terminals. The MATRIX TYPE with 7 or 8 terminals WILL NOT WORK. With a 12-key pad - over 10 000 different codes are available. If you need a more secure code - use a bigger keypad with more "Wrong" keys wired to "F". A 16-key pad gives over 40 000 different codes.

The Support Material for this circuit includes a step-by-step guide to the construction of the circuit board, a parts list, a detailed circuit description and more.

Veroboard Layout

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