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When you say Miniature, I assume that you mean smaller than human sized?
If that is true, the sensors depend on what you expect to ‘trigger’ the doors to open.

The classics are:
          Photo transistor:  When exposed to light it will conduct.  By covering it (making it dark) it will conduct much less. 
Cds Cell: Similar behavior, Acts like a resistor that is sensitive to light.
Hall effect sensors.  If the item being sensed is metal (Fe) based.
Fet-Touch sensors:  Very high gain sensors that detect the static discharge of  human touch.
Capacitive sensors: Good for sensing just about anything except they have a very limited range.  (usually millimeters)

Some other devices that MAY NOT be useful due to the model being ‘miniature’

Ultrasonic: I have not seen a micro-Ultrasonic setup but I suppose it could be done.  I think the trick would be building a pickup that can detect the weaker ‘clicks’

PIR (passive Infrared) This usually only works on Living/Warm blooded things.
It is the same concept as the Photo transistor except it detects HEAT from a body.
Some are so sensitive they can detect changes in air temp but usually only over a large area.  It could work but it would be difficult to use in a very small area.

Pressure mats:  These are often expensive if you buy them but I have seen very functional ones built from foil and paper. They normally require at least 50lbs to trigger.

Let us know more about the size of the model and what will trigger the doors and I am sure someone will have a clever idea to help.


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im only building a small model house with doors that are 2x3 inches in size.
Im planning that if someone will get near the door it would automatically open, so most likely it would detect human touch, or maybe light. But since this is a miniature, fingers would serve as the actual size of the person.

Mike-  about the Fet-touch sensor, can you explain to me how it would work and the configuration on the device. Any specific name for that sensor once ill buy it..?

Another thing is im wondring if whats the best way to reverse the rotation of a servo.... any suggestions...

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Zoinks, that’s a lot of questions.
All right 1 at a time:
    Touch sensors.  A FET touch sensor is the same thing used on those lamps that you touch to change the brightness.  IT can be done with any high gain amp.  If you have ever touched the input to an audio system and heard that loud hum noise, you know how it works.  The input is sensitive and can pick up the touch of a person.  But rather than drive an audio signal it just flips a relay.

Another touch type switch is called the Q-Prox.  It is a single chip solution that uses a simple wire coil to detect a touch.  I have included a link to the part.  They have other chips that offer multiple inputs (2 keys,4 keys ect)

On the issue of Reversing the Servo.  I guess I would need to know how you are making it go forward in the first place.

While this is not the normal usage I have seen a very simple application of R/C Servos where the Power and ground leads were connected and the signal (White or yellow) wire was simply connected to +V to rotate the servo to full right and ground to move it full left.  The servo normally looks for a 1.5ms pulse every 100ms (max) to maintain a position of



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The 556 chip is very similar to a pair of 555 timers.
On the left side of the chip the Cap and resistors are used to set a frequency for the signal.
Usually this frequency is not needed to be exact.  Most servos are forgiving but it should be around three thousand pulses per a second or 3 Khz. Use the adjustable resistor to move up and down the range until your servo starts to respond.

The Cap and resistors on the right ser the Pulse width.  While the left side of the 556 is acting like a simple pulse generator, this side is wired as a “One-shot” with every pulse that the left side creates , the right side generates a pulse whose length is determined by the capacitor and resistors.  The switch at the top of the circuit shorts out the 10K resistor.  When the switch is open the resistance is such that the pulses generated are much longer and this causes the servo to turn all the way one direction.  When the switch is closed, the 10K resistor is effectively ‘taken out’ of the circuit.  With a lower resistance the pulses now become smaller and the servo changes direction.

There are 2 “extra” capacitors in the circuit. On pins 3 and 11. these are needed to keep the circuit stable.  While they do play a role in how the circuit works, if you are a beginner, focus more on the resistors and capacitors on the outside edges of the circuit.

Good Luck!

(PS.  Some servos are designed for speed, others are made for power.  The more powerful ones are usually slower also.  If you want your doors to ‘slowly’ open you can use a high torque/low speed servo.  The other way to accomplish the speed control is to alter the speed that the resistance of the 10K resistor is changed.  This is a bit more complex but you might enjoy the challenge of figuring it out.)


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