FISHERY BULLETIN: VOL. 72. NO. 2 



telemetry, particularly for work in the open sea. 

 It is not possible to send across oceans with a 

 miniscule device. But almost any small amount 

 of energy will work in laboratory tanks. The low 

 power transmitters described here are useful at 

 distances of at least 100 m. 



CONSTRUCTION 



Heartbeat Transmitters 



The cylindrical form of our devices is dictated 

 by the transducers and batteries, both of which 

 are round. A stainless steel tube is chosen into 

 which the battery fits. The electronics are then 

 packaged to this inside diameter. They are cast in 

 epoxy resin in one end of the tube, with the 

 heartbeat lead or thermistor coming out of the 

 plastic. The tubular transducer is fit either inside 

 or outside the tube, and similarly embedded in 

 plastic to assure electrical insulation. The bat- 

 tery is held in place with a watertight cap on the 

 other end. The metal case forms the indifferent 

 reference electrode to the one placed near the 

 heart. With the exception of the transducers 

 (Penn Engineering & Manufacturing Co. Inc., % 

 Aquadyne, Inc., Falmouth, Mass.) parts used in 

 these devices are routine. Parts for a heartbeat 

 transmitter cost about $15.00. 



Large Heartbeat Transmitter 



A vertebrate heart produces an electric field 

 when it beats. A millivolt level signal from an 

 electrode near the heart is amplified by Qi in the 

 schematic of Figure 1. The larger voltage is used 

 to vary the frequency of an oscillator (Q2 and Qs). 

 Another amplifier (Q4 to Q7) after the oscillator 

 drives a transducer at this frequency producing 



i.z.j.i.s.e. 



2N5I38 

 2N5133 



T, — TRANSDUCER, PZT-4 CERAMIC CYLINDER 2.25 cm 0,D. 

 FREQUENCY ^- 50I.HZ ',;|7cmL0NG 



FREQUENCY DEVIATION = 200hz /mv lSJEK G INPUT 



Figure 1. — Large heartbeat transmitter. 



sound in the water. With a carrier frequency of 50 

 kHz the typical excursions are a few hundred 

 hertz. Thus the EKG voltage is transformed into 

 variations of the sound frequency. Another gain 

 stage before the heartbeat amplifier can make 

 the transmitter sufficiently sensitive to send 

 signals of 100 /uv or less. In this way we have been 

 able to follow the electromyograms in the red and 

 white muscles of fish. Figure 2 is an example of 

 an EKG recorded from free swimming Atlantic 

 cod, Gadus morhua, and Atlantic salmon, Salmo 

 salar. The various sequential details of the 

 heartbeat are clearly shown. The transmitter is 2 

 cm in diameter and 8 cm long. 



Small Heartbeat Transmitter 



Similar performance at the cost of greater ef- 

 fort at miniaturization can be had with the sim- 

 pler and smaller transmitter shown in Figure 3, 

 which is 1.5 cm by 7 cm. Reduced power consump- 

 tion allows the same battery life (3 wk) as the 

 larger transmitter. Be replacing Ra in either 

 transmitter with a thermistor, temperature will 

 control the carrier frequency. The heartbeat can 

 still be transmitted as variations around this 

 changing frequency. 



Depth Transmitter 



The depth of a fish has been a difficult variable 

 to transmit because of the lack of practical pres- 

 sure sensors. Some information about depth has 

 been discerned from water temperature. The re- 

 cent appearance of small sensitive silicon pres- 

 sure sensors has made direct measurement of 

 depth feasible. We have built a depth transmitter 

 around such a device (Figure 4). 



The DC output voltage from the pressure sen- 

 sor is increased by an operational amplifer, A. 

 These larger voltage excursions control the fre- 

 quency of an oscillator in the same way as amp- 

 lified heartbeat signals. The thermistor in Qi 

 compensates the oscillator against frequency var- 

 iations due to temperature. The resistor in paral- 

 lel with this thermistor must be empirically cho- 

 sen to optimize this compensation. This allows 

 the received frequency to be interpreted as pres- 

 sure. The frequency change is 1000 Hz/m of depth. 

 A 20°C change in temperature causes an equiva- 

 lent pressure error equal to 5 cm of water. The 

 circuit is a voltage controlled oscillator, useful 

 with any millivolt-level DC signals. This instru- 



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