variable frequency output using a looped circuit 1

[tomdickinson]

the circit that im designing is for a reversing aid.
the reversing aid uses the tried and tested ultrasonic approach.
i have a circuit whereby the final output is either high or low from a comparator depending when a ultrasonic reflection has been detected.
i need something added to this circuit to make a loop where by the closer the reflected ultrasonic signal is the higher the frequency and the longer the reflected ultrasonic signal takes to be recieved the lower the frequency of the circuit.
ive been racking my brain and im sure that a flip flop must be used but cant figure out where and with what logic.
i really dont want to use a voltage to frequency conveter as this will make the system less accurate.

thanks for any help
tom.

 

[BrianG]

More details are needed about the ultrasonic transmitter and the output from the comparator circuit.

Does the transmitter send out a) a short pulse or b)a continuous level? What does the output from the comparator represent when it detects a reflection, i.e. how does it indicate a varying distance?

The comparator needs some form of discrimination to indicate distance: a simple "high or low" output as you described cannot do this.

If the transmitter is pulsed and the comparator gives a pulse out in response to receiving the reflected signal, then you can only detect obstacle distance by relative time interval measurement- just like radar. This can be a relatively complex way to measure the distance accurately. (Do the sums for the velocity of sound in air. Note that this varies with all sorts of things like temperature and altitude)

If your comparator is arranged to give a varying pulse width with increasing reflection strength, i.e. wider pulse the nearer to the obstacle, then this is much easier to process to get the result you require.

 

[tomdickinson]

There is a 40kHz timer fed into an inverter to give increased square wave amplitude before entering the UST.
The Signal is detected by the USR and then amplified by 1000 rectified then using a diode pump to give a varying voltage which the comparator detects and outputs a high or low signal.

The design originally incorporated a short bust of 40kHz ultrasonic but found that this was detected less easily by the USR.
As you say a comparator cannot do the task that I require as it gives either an on or off signal not a varying frequency.

What is required is a system where by the reflected signal is detected, outputting a pulse for a short period of time and then resetting the circuit so that another pulse can then be detected in a loop. This loop might then be able to generate a frequency that would change depending upon the time taken to receive the reflected signal.
This varying frequency can then be outputted into a microprocessor with a pre written program which will be used to display the distance on an LCD.
If this sounds plausible then any comments would be gratefully received.

The comments about the varying pulse width might be an easier way achieve the desired effect. How might a comparator be made to give a varying pulse width? Is this a standard method? And does this involve pulsing the 40kHz wave into the transmitter?

 

[cbarn24050]

Hi, there are 2 basic ways to do this. Transmit a pulse and measure the time it takes to return. Apply low frequency modulation to your tramsmitter, recover it from the recieved signal and measure the phase shift. At these low frequecies any cheap microcontroller will do for either method.

 

[Heydave]

Use a SR flip flop. It would initially be set with say an output from a power supply regulator chip to initiate transmission and reset when a reflection is detected. At this point, a delay circuit would then set the flip flop again thus starting over. The flip flop is made from either two NAND or two NOR gates depending on the logic state from your detector and power reset. The delay circuit could be formed from a remaining gate with an RC. Simply feed the output of each gate to one input of the other gate to form the flip flop with complementary outputs. The set state of the flip flop would trigger transmission setting the flip flop and restarting transmission again until detection. The total parts, not including the ultrasonic device, consist of a quad gate and regulator with a few descrete resistors and caps. Pipe either output of the flip flop to a sound system.
The speed of sound is about 1000 ft/sec and you must consider the round trip. 1 Hz would be 500 feet not including the delay circuit effect of lowering the frequency. 1 foot would be less than 500 Hz depending on the delay circuit timing. The delay circuit timing will depend on how fast the detector can detect the reflected sound. This analysis might be useful in determining whether this direct approach is adequate for the intended application or if you will require a warning tone created by say a microcontroller in response to a measuring circuit.

 

[BrianG]

It seems that you are trying to modulate the wrong end of the circuit with your "loop". Generating any form of pulses from the comparator cannot produce information which is not there!

As stated in earlier posts you need to measure the time interval between transmit and receipt of echo (just like radar) to work out the distance. The "pulse" to your microprocessor must come from the receiver / comparator circuit in response to an echo (obviously!) but this will require a controlled transmitter pulse to produce a specific burst of energy.

I note that you had tried pulsing the transmitter drive before, but you state that this was not very successful. What duration was the pulse and did you make any measurements for how long the receiver takes to respond to a reflected signal. You may need to have quite long transmit pulses with a low repetition rate to get the receiver to turn on and off cleanly. However, if the response time is very slow this may limit the viability of the whole system over the range you require due to excessive "dither". (This is the uncertainty in the time measurement taken from the end of the transmit pulse and the receiver output pulse.)