Analogue Proximity Sensor

What distance do you consider?

An IR based sensor could generate an analog signal, but probably not a very linear one. Also the signal will depend on factors like reflectivity of the object to be sensed an environmental lighting.
A radar based sensor could be used, but the ones I've seen typically are set up for digital (threshold based) operation. It may be possible to modify such a sensor to give an analog signal.
For very short distances a capacitive sensor can be used. However, the signal will depend on the conductivity of the object to be sensed.
Although the web page for this ultrasonic sensor mentions the use of a microcontroller for measuring distance, I don't see any reason why you shouldn't be able to use this sensor in an analog setup. I imagine this setup:

• a clock source to repeatedly trigger the emission of a sound burst (ping), e.g. as simple as a 555 timer.
• a constant current source charging a capacitor where the capacitor is discharged by the triggger impulse for the ping-emitter
• a track-and-hold circuit that follows the voltage on th capacitor and is triggered by the ping-receiver

The voltage on the capacitor will be reset to 0V on emission of the ping. It will then rise linearly with time due to the constant charging current. Once the ping's echo is received, the receiver triggers the track and hold and the voltage at the output is directly proportional to the time from emission of the ping to reception and therefore to proportional to distance. No microprocessor required

Edit: If you can live with a discontinuous output voltage, you could simply turn off the current source (instead of using a separate track-and-hold). The capacitor will hold the voltage until it is discharged for the next measurement.

 

Argh, again 4 minutes

 

I think radar or ultrasound would be ideal for this range (just an opinion). An LM3914 will control an LED bargraph display from an analog input voltage that can be derived as Kris and I described above.

 

Good job, Frenoy.

@Gryd3: If you want to build your own sensor electronics anyway, here's how it can be done:

• The 555 generates short pulses spaced approx. 20ms apart.Each pulse discharges C3 which will then be re-charged by R6. I used a simple resistor instead of a current source. This introduces a certain non-linearity which may not be important for your project.
• B1 is a behavioural source to model the delay of the returning ping. The ".step" and ".param" statements compute the time delay for distances of 0.1m...0.5m in increments of 0.1m. The resultinmg signal is shown as light blue pulses (output of B1).
• Switch S1 and capacitor C4 form a primitive sample and hold circuit.
• The red trace in the top diagram is the voltage on C3. The non-linear charge curve is clerly visible.
• The dark blue traces show the sampled voltage which varies with distance but otherwise stays rather constant.

Note that the top diagram shows the superposition of 5 simulations, 1 for each distance. The variation can be seen in the dark blue traces only.