Re: Need advice on converting high frequency to low frequenc
Hi,
Yes, I can do that. I hope you know about op-amps. Pin 3 of the IC is the -ve input of its internal opamp and pin 4 is its +ve input. As per data sheet, the IC produces its highest frequency, depending on the capacitor value, for 1mA current in the feedback transistor.
In your circuit, you have applied the sonar output to the positive input, pin 4. This will make pin 3 of the opamp to rise to the sonar voltage, since op-amp requires only a very small differential voltage across pins 3 and 4 to produce an output. Since you have connected a 4.8k resistor from pin 3 to ground, at 4.8V of the sonar output, a current of 4.8V ÷ 4.8k = 1mA will flow from pin 3 to ground, which will come from the collector-emitter of the feedback transistor. A good opamp does not take any current into its inputs. so, when your sonar produces 4.8V, you get maximum frequency and when sonar output is almost zero, the current through the transistor and hence the frequency generated by the transistor is the lowest. Now you wanted to reverse this condion, that is to generate 1mA current when the sonar output is minimum and viceversa.
In my modified circuit, the sonar connection is moved to the negative input of the opamp pin 3, through a 4.7k resistor. The negative input in this connection works as a virtual ground, since the +ve input is grounded through a 2.4k resistor. So, when sonar output is say zero, no current flows into pin 3 from sonar , but due to the -5V connection, a current of 1mA approximately (5V÷ 4.7k) flows to ground from pin 3. IC will now produce the highest frequency. Next, when the sonar output goes to say 5V, it produces a +ve current of 1mA into the virtual ground point, but since the same current is taken away by the -5V supply, the net curret supplied by the feedback transistor is zero, which means the IC will produce the lowest frequency.
BUT, HERE IS A CATCH, THE DESIGN ASSUMES THAT YOUR SONAR OUTPUT IMPEDANCE IS SMALL AND THAT IT CAN SUPPLY 1mA OF CURRENT AT 5V WITHOUT GETTING LOADED, YOU CAN CHECK IT BY CONNECTING A 4.7k RESISTOR TO GROUND IN YOUR PRESENT CIRCUIT FROM SONAR OUTPUT AT THE HIGHEST FREQUENCY TO SEE IF THERE IS CONSIDERABLE FALL IN OUTPUT VOLTAGEOF SONAR. IF IT IS THERE, YOU WILL NEED A BUFFER OPAMP TO BE PUT.
I still feel that your original circuit is the best and you only measure the frequency with a counter. Alternatively did you try if the sonar has any jumper position to provide (1-x) type output.
Regards,
Laktronics