BAT DETECTOR

1. BAT DETECTOR Student: S. FRENEHARD Supervisor: J. L. ERRINGTON All rights in this work are retained by J. L. Errington

2. Contents 1: ABOUT BATS. 1.1: BATS. 1.2: DIFFERENT SPECIES. 1.3: MOVE IN COMPLETE DARKNESS. 2: PROBLEMS OF OBSERVING BATS. 2.1: DETECTING A BAT. 2.2: COMPLETE DIAGRAM. 3: THE BAT DETECTOR CIRCUIT. 3.1: AMPLIFICATION. 3.2: DETECTION. 3.3: FREQUENCY DIVISION. 3.4: AMPLITUDE RESTORATION. 3.5: POWER AMPLIFICATION. 1

3. 1: ABOUT BATS. Bats are the only flying mammals and are found throughout the world. They are furry, intelligent and social, spending hours grooming daily. their wings are skin supported arms and elongated fingers. They do not breed until their second to fourth year and produce a single baby, but not necessarily every year. Mating occurs in autumn or winter this being the best time to use the bats detector to its full capability, but the females do not become pregnant until June or July. The baby is raised with great care by its mother and is suckled frequently both by day and night. At three weeks the baby can fly and after five weeks is weaned. Bats can live for up to 30 years. 2

4. Soon after the dinosaurs had died out mammals began to diversify and expand their range. It is believed that some type of insect-eating mammal which scrambled around in trees gradually evolved to become the type of bats that we know today. The oldest bat fossil discovered so far is approximately 50 million years old. Since these early times fossils show that the bats became more and more diverse as they adapted to different climates, habitats and foods.

5. DIFFERENT SPECIES There are about 900 and 1000 different species of bats in the world. There are big bats with a 2 metre wingspan; small bats with bodies only 3 cm long; heavyweight bats over 1Kg ; light bats of 2g , white ones , striped ones, naked ones and even ones with fluffy head-pieces! With such a large number of species, it is sensible to arrange them in some sort of order. Bats ( chiroptera is their scientific name ) divide into two sub order the Megachiroptera and the microchiroptera. The former are the “old world” fruit bats ( ranging from Africa eastwards to the pacific islands ) and generally have very large eyes, which they use for navigation in place of echo location and have teeth designed to cut and small eyes and many use echo location to navigate and to find food; they include a very large number of species world-wide with many differences so they are divided again into 17 families. The largest family contains over 300 species ( one third of the number of bat species in the world ). 3

6. THEY MOVE IN COMPLETE DARKNESS. Most species of bats rely on echo location to guide them at night and to hunt for their prey. This echo location uses ultrasonic frequencies to resolve small objects. Bats have developed specially shaped noses and ears to help them emit and receive the ultrasonic sounds. 4

7. Echo location Echo location enables bats to find their way around in the dark and to pinpoint the location of flying insects. So simply the bats gives out a short, load shout and by carefully listening to the echoes of sound that is bounced back from buildings, trees, the ground, insects and anything else in the area can determine where it is. The echo will come back almost immediately from close objects, and will be clearer than echoes received from waving tree leaves for example. The bat can build up a picture of the position and distance of objects around it, their movement, and also something of their texture, from the echoes it picks up.

8. Echo location (2) Bats call about ten times every second, and in turn listen to the series of echoes. If an echo seems to come from a slightly different direction relative to the bat after each call then it is obviously a moving object. In practice the calls are very complex and made up of different ultrasonic frequencies. Some parts of a call give information about substance of the object. It is a very sensitive system and can easily locate very small insects, or for example wires stretched across a fence, in pitch black conditions. Echo location calls are very loud, yet we can not hear them, due to the ultrasonic frequencies involved being above the human hearing range. This is why a bat detector is needed, to allow us to hear the ultrasonic signals. 5

9. . 2: DETECTING A BAT. As we have seen before the bat emits ultrasonic soundsbetween 20 kHz and 160 kHz which is above the human hearing range of between 20 Hz and 16 kHz. However if you divide this frequency by 16 you obtain a audible signal for the human potential. The main idea of my project is to make the bat’s soundaudible by humans. 20 Hz 16 kHz 20 kHz 160 kHz Human hearing Frequency range used by bats Frequency (kHz) 16 kHz 140 kHz Differences between human and bats hearing 6

10. BLOCK DIAGRAM OF BAT DETECTOR To divide the frequency of the bats calls we must firstly receive and amplify these signals. We will use a piezoelectric transducer which has a good frequency response at 40 kHz, and a system to convert the ultrasonic waveform into something humans can perceive. We have tried to reproduce the same signal that the bats emit. In order to do this we must reduce the frequency, and then restore the amplitude of the signal. AMPLIFIERS COMPARATOR FREQUENCY DIVIDER 16 2500 HYSTERESIS ULTRASONIC TRANSDUCER MIXER DETECTOR Ge POWER AMPLIFIER LOUD SPEAKER 2 Watt DIAGRAM OF THE BAT DETECTOR 7

11. 3: THE CIRCUIT 8

12. Circuit description • The ultrasonic sound is received by the piezo transducer, which provides a signal to the amplifier stage. • The amplified signal goes to the detector, which provides a measurement of the amplitude. • It also goes to the comparator, which produces a square wave signal for the input of the frequency divider. • The frequency divider converts the square wave to a square wave at lower, audible frequency. • The output from the detector is mixed with the output of the divider which allows the restoration of the envelope of the signal. • Finally this signal feeds the power amplifier producing an audible sound for the human hearing. You will find the complete circuit diagram on the following page.

13. Ultrasonic Transducer +9V 2.2pF 3.3pF 420p 1M 20K 470K 100nF 1K 6 9V - +9V 10K 2 10K Ge 3 8 7 10F 680k LF353 - 20K + 8 1 1 LF353 LF353 + 5 1n - + 4 4 2 3 1N4841 0V 10K 1K 0V 100nF 100K 100nF 470F 10k 100k 100nF 10K 100K +9V +9V 10K 16 6 Output to power amplifier - 7 2 +9V 10 7 - LF353 6 /16 + 3080 5 5 + 5 3 11 4040 4 10K 1N4841 4.7nF 1nF 1M 8 D FET 0V 1N4841 S G 10K 3.9k BAT DETECTOR CIRCUIT

14. Choice of components • The circuit had to work from a single PP3 battery • Various operational amplifier types were considered, but the bandwidth requirement led us to choose a dual low noise op amp: the LF353 • The circuit configuration chosen needed a split supply

15. Supply splitter The first section of the amplifier uses one half of a LF353 as a voltage follower to create a split supply. This provides a voltage level that allows 4.5 ~ 0 ~ - 4.5 operation. 9V LF353 9V 10K 8 3 4.5 Volt 1 + - 470nF 4 0V 2 10K 100nF 4.5 VOLT REFERENCE 9

16. Input stage This diagram shows the supply splitter and input amplifier with a gain of 50 and 3dB bandwidth limit of 80kHz (the G-b product of the LF353 is 4MHz) Piezo sensor +9V 2.2pF 420p 1M 20K 9V 6 1K 8 - 7 LF353 1 3 + LF353 + 5 1nF - 4 20K 2 470mF 1K 0V 100nF The ultrasonic sensor feeds the second half of the LF353 with the high frequency signal that the bat emits, which is in the region of 20KHz to 160KHz. You can see below the frequency response of the ultrasonic transducer.

17. Frequency response of sensor SMALL EFFICIENT BANDWIDTH 0dB 10dB Frequency (kHz) 20dB 160KHz 0Hz 38KHz 40KHz 42KHz FREQUENCY RESPONSE OF THE PIEZOELECTRIC SENSOR This signal from the ultrasonic transducer is filtered by a first order high pass filter which has a cut frequency of 20KHz. We wanted to have a very high gain, yet wide bandwidth, so I have decided to use two amplifiers with a gain of 50 each. Common amplifiers have a gain-bandwidth product of 10MHz or less, so a gain of 2500 would only allow a bandwidth of 4kHz! As you can see in the diagram on the following page the gain is set by the value of the feedback resistors.

18. First stage amplifier The stage gain is controlled by the feedback resistor between the pins 6 and 7, and the input resistance Rin. This is the sum of R2 and the transducer’s impedance. For the calculation that follows we assume this is low compared to 20K. Gain = - R1 / R2 Here R1 = 1000K, R2 = 20K so Gain = - 50 Fc = 18KHz gain Frq Ultrasonic Transducer 20dB /dec 2.2pF 420p High pass filter 1M Fc = 1 / 2 πR C Here R = 20K, C = 420 pF hence Fc = 18KHz 20K R2 R1 6 - 7 LF353 + 5 20K The high pass filter uses 18KHz as the lower frequency limit to prevent interference from audible sounds. 11

19. Second stage of amplification: The second amplifier also has a gain of 50. The signal from the first stage is again filtered by a high pass filter which has a cut frequency of 16KHz. Gain = - R3 / R4 Here R3 = 470K, R4 = 10K Gain = - 50 gain 3.3pF Fc = 16KHz Freq High pass filter 470K 20dB / decade 10K Vin +9V 2 - 8 1 1nF LF353 Vref + 4 Fc = 1 / 2 π R C Here R = 10K, C = 1000 pF hence Fc = 16KHz 3 0V 10K The output from this amplifier is fed to a comparator to provide a square wave for the frequency divider; and also to the detector which records the signal amplitude to be restored later. 12

20. Ultrasonic Piezo Transducer +9V 2.2pF 3.3pF 420p 1M 20K supply splitter 470K 1K 100nF 6 9V - +9V 10K 2 to detector 3 8 7 LF353 - 20K + 8 1 1 LF353 LF353 + 5 1n - + 4 4 2 3 reference ‘ground’ 0V 10K 1K 0V 100nF 100nF two amplifier stages, with gain = 50 approx. each stage 100nF 10K 470F comparator with hysteresis 10K 6 - 7 square wave output LF353 + 5 100K 1M AMPLIFIER CIRCUIT WITH COMPARATOR

21. Amplitude Detection A passive detector was used because the amplitude of the signal is now big enough to allow this. A germanium signal diode allows any signal bigger than 0.25V to be recorded. This provides better dynamic range than a silicon diode. 0.3 Volt High pass filter = RC Ge 10K 100nF 1N4841 10F 680k A = RC T constant  13

22. Detector buffer A FET buffer matches the high impedance output of the detector to the low impedance input of the 3080. If you connect a low impedance to the detector you change the value of  +9V DIODE OUTPUT D FET S G 10K 3.9k 14

23. Frequency division The divider gives a frequency division of 16 (2 to the power 4), so an input signal at 40kHz is mapped to 2.5kHz. A comparator with hysteresis rejects spurious signals. Reference ‘ground’ 10K 100K input +9V 16 6 - 10 7 LF353 /16 + 5 5 11 4040 10K 100nF 4.7nF 1M 8 0V Evaluation of the hysteresis. The minimum output from the comparator is 1V away from the supply 1V 8V 1M 1M 3.5 3.5 4.5 + = 4.505V 4.5 - = 4. 495V 110 110 10K 10K 4.5V 4.5V So we have a hysteresis of 4.505 - 4.495 = 0.01Volts 15

24. Calculate comparator hysteresis Reference ‘ground’ 10K 100K +9V input 16 6 - 10 7 LF353 /16 + 5 5 11 4040 10K 100nF 4.7nF 1M 8 0V The output from the comparator swings to within 1V of the supply: so it can be 1V or 8V approximately. These two possibilities are shown below, and their effect at the input calculated. 1V 8V 1M 1M 3.5 3.5 4.5 + = 4.505V 4.5 - = 4. 495V 110 110 10K 10K 4.5V 4.5V So we have a hysteresis of 4.505 - 4.495 = 0.01Volts

25. Diagram showing effect of hysteresis Good signal Spurious signal Good signal Spurious signal 0.01Volt HYSTERESIS The received sound is not a pure wave, and without hysteresis the signal that feeds the frequency divider would be corrupted as can be seen above. The frequency is then not the same that the bat emits and you have at the output of the divider a bad translation of the original sound.

26. Amplitude restoration We wanted to impose the amplitude of the original signal on the divided signal, which was now at constant amplitude of about 8V. The CA3080 Operational Trans-conductance Amplifier was used. This has an amplifier bias input which provides linear gain control. The two diodes are necessary to provide some bias offset. +9V 10K 4.5V reference 100K output to audio amp. 7 2 - 6 100k signal from frequency divider 3080 + 5 3 4 10K 1N4841 1nF 1N4841 amplitude signal from FET buffer 10K The current Iabc which feeds pin 5 controls the gain of the amplifier (Iabc must not exceed 2 mA).

27. RESTORING THE AMPLITUDE COMPONENT Frequency divided signal 4040 Frequency divided signal with amplitude restored Amplitude of sound from bat DETECTOR

28. THE AUDIO AMPLIFIER A bridge tied load amplifier configuration was used, using a pair of LM380 amplifiers. This can dissipate more power because it uses two amplifiers working in push-pull. U = 4V 2 U P = P = 2 Watts R R = 8 Ohm + + LM380 LM380 - - DIAGRAM OF THE POWER AMPLIFIER You can see the complete diagram on the following page. I have used several decoupling capacitors to avoid oscillation of the circuit, which now works properly.

29. POWER AMPLIFIER BRIDGE P 1M OUT OF 3080 9V F 1 8 7 14 LM380 9 6 2 LM380 13 100nF 100F 10k 10 5 1nF 3 12 11 4 4 11 12 5 3 10 TO DETECTOR SUPPLY 13 6 9V 2 9 14 7 8 1 100 470nF 100F 100nF ZOBEL NETWORK 2.2 100nF 8 OHM SPEAKER

30. Power amplifier notes • A bridge tied load configuration gives 4X more output power from a 9V battery than a single ended amplifier. • It also gives a more even load on the battery and less disturbance of the supply voltage • Layout and power supply decoupling are important.

31. Project Evaluation • All parts of the circuit worked well as separate modules. • Because of the high gain of the detector it isn't possible to use a loudspeaker as the output. What happens is that harmonics in the output from the speaker are picked up by the detector causing feedback oscillation. • Similarly when the output amplitude restore was connected the circuit went into uncontrolled feedback. We were unable to resolve this problem.

32. Success! The circuit works well as a simple frequency division detector if the amplitude restore components and audio amplifier stage are omitted.