These are some ideas for frequency division bat detectors. These work by dividing the incoming frequency by a fixed factor.

Two types are described here:
Simple division detectors output a fixed-amplitude signal, so it is hard to judge the distance to the bat.
Amplitude-retaining division detector output a signal whose amplitude is related to the input amplitude. The distance to the bat can now be judged by listening to the loudness of the signal.

# Simple frequency division ideas

Bat signal are analog signals, continuous in time and in amplitude. To drive a frequency divider chip, like a digital counter, you need a digital signal instead of an analog one.
To convert from analog to simple digital (on or off), you can use very high gain amplifier to make the signal so large that digital electronics can trigger on it.
Another way is to use a comparator, which can directly convert a signal of some 10's of millivolts into a full digital signal. A possible circuit for that is described below.

### DC coupled AC comparator This comparator circuit can be directly (DC) coupled to a previous processing stage. The DC component is separated from the input signal by means of the low-pass filter and appears at the (-) input of the comparator. The (+) input sees the unfiltered signal (DC + AC). The result is that the comparator only triggers on the AC part.

A little hysteresis by means of the high-value positive feedback resistor makes it insensitive to small noise signals.

A LM393 seems very well suited for a bat detector, because it is easy to find and has low power consumption. It has an open collector output, meaning that it can only pull the output low. The 10k resistor pulls the signal high during the times that the comparator does not trigger.

# Amplitude retaining circuit ideas

An amplitude retaining circuit maintains the original amplitude of the bat call in the divided down version, so that you can judge the distance to the bat for example.

### Chopping Technically speaking, this can be implemented by generating a signal that represents the amplitude (or envelope) of the bat signal and multiplying this signal with the output of the digital divider.
Multiplying with a digital signal (1 or 0) simply means that the input signal is either passed (multiply by 1) or not passed (multiply by 0). This can be very conveniently implemented by using an analog multiplexer, the CMOS 4053 (pdf)for example. One analog input is connected to the amplitude signal and the other to ground. The state of the switch is controlled by the divided-down signal.

### Sinewave approximation An new idea that I haven't tried out yet is shown on the left.
Instead of just chopping the envelope (amplitude) signal, the signal is tapped by means of a resistive divider and an 8-to-1 analog multiplexer. Each tap represents the value of the sine function at 45 degrees intervals. Every time the counter value increases by one, the next tap is selected, so after 8 steps, a full sine approximation has been made. The counter consists of the least significant bits of a 4024 CMOS IC (pdf) for example. The voltage at the top of the resistive divider determines the amplitude of the sine approximation. By feeding the signal of an envelope detector here, we have a nice amplitude- retained sinewave output with only a modest amount of components!

### Another idea for an amplitude retaining detector Use a comparator to determine the polarity of the incoming bat signal. The output of the comparator switches the bat signal to one of two low-pass filters. The low-pass filter outputs now represent the average level of the positive and negative parts of the bat signal. The output of the comparator also goes into a divider-by-16. The divided down signal operates another switch that chooses between the two low-pass filters.
In other words: using a comparator, an electronic switch and two low-pass filters, an amplitude detector is constructed. The output of this amplitude detector is chopped by another electronic switch at a rate of 1/16th of the bat frequency.
The advantages of this circuit are that (1) only a small number of components is required, (2) the envelope signal is symmetrically chopped and (3) no low-drop diodes are required. The two switches could be provided by a single 4053 CMOS (pdf). The comparator can be a LM393.
The low-pass filters can probably be implemented with simple RC-filters.

# Frequency division by mixing

One of the detectors of J.P. van Dijk uses a rather unique design for amplitude retaining frequency division. The bat signal is converted to a square wave of a frequency of 0.9 times the bat call fundamental frequency. (Is it possible to lock a PLL to the rapidly varying frequency of a bat call?) The square wave is used as the oscillator signal in a mixer that multiplies it with the original analog bat signal. This results in a signal containing a difference frequency of 0.1 times the original bat call frequency. The amplitude is automatically retained because of the mixing operation.