Commercial receivers often have a digital frequency display (LCD- or LED). These display the internal center frequency of the receiver, not as some might think the frequency of the incoming bat call. Some displays even claim to show frequencies in resolution of up to 0.1 kHz which is quite pointless as bats mostly use a large spectrum of frequencies in their calls and cannot be heard on a single frequency, much less can this be used to determine their species. Moreover a heterodyning receiver does not use just one frequency but a range often streching to a width of about 9 kHz.
Still it is helpful to know which center frequency the receiver is tuned to. Given the above limitations it becomes self-evident that for most uses markings in steps of 10 kHz are sufficient. Therefore a detailed display is really redundant and expensive as well as battery-consuming and demanding for the display case. This is why the added circuit board for the NE612-receiver signals these steps acoustically by beeping at 20,30,40,50,60 and 70 kHz which is mixed into the output signal for the speaker, reducing the effort to a small processor and 4 cables to the receiver. Little time is needed to adjust to this method and learn to use it, for example to turn to 30 kHz you would turn the knob left to the end, then right to the second beep, first beep is 20 kHz, second 30 kHz.
This is really the prototype of the timer marker, but the frequency counter resembles it except for two additional resistors. I have the circuit boards on order and it will be possible to construct the counter for the NE612 soon, unfortunately the counter does not yet work with the TCA440-receiver.
A small pushbutton can be added to the circuit board to have the processor signal the current frequency when the button is pressed, giving the value in two digits in Morse code. Single beeps tend to become difficult to count and Morse numbers are easy to learn: 34 kHz register as beeps "...-- ....-" .
Depressing the button longer will swich the processor to mute, which is useful when scanning across frequencies more quickly for a bat and constant beeping would irritate. Another longer depression will activate the signals again.