This article shows you how to make an electric fish toy that swims by moving its tail, pulled by two electric magnets.

The magnets are switched ON and OFF by a relay oscillator. The frequency of oscillation is reciprocal to the charging capacitor capacitance value (Farads). The DPDT (Double Pole Double Throw) relay has two switches. One is used to maintain the oscillation and the second to supply power to the two magnets. I used a 9 V battery to power the 12 V relay oscillator and one 1.5 AA battery to power the magnets. I needed a separate power source for the magnets because the resistance of the magnets was very small and connecting the magnets to 9 V battery would not only drain the battery but also interfere with the relay oscillation circuit. It might be possible to implement this design if a smaller (low) voltage relay is used.

I used a 1.5 V D battery harness and placed my AA battery in a special white plastic cylinder (that you see in the photo) because I did not have a D battery available.

Supplies

Components: packaging foam/plastic bottles/wood, thin insulated coil wire, two nails (to make the magnets), screw/nail (that will be pulled by the electric magnets), tape (making/clear), DPDT relay (low voltage/low power), 9 V battery, 1.5 V AA/AAA/C battery, solder, 1 mm metal wire, 9 V battery harness, 10-ohm resistor (high power), high-value electrolytic capacitors - 5 (470 uF, 330 uF or 220 uF).

(I suggest that you used rechargeable batteries because both the relay and the magnets will quickly discharge the batteries).

Optional parts: 2 mm hard metal wire (insulated or non-insulated), 1.5 V battery harness/plasticine/rubber band, general-purpose/high power diodes.

Tools: Soldering iron, pliers, stone/sandpaper (to remove the coil wire insulation).

Optional tools: multimeter, voltmeter, USB oscilloscope.

Step 1: Design the Circuit

The components shown inside the DPDT box represent the relay model. You only need to obtain the components that are outside the box.

The two diodes are included as part of the relay model. However, a typical mechanical relay does not have diodes connected between the switching contacts. I just included those diodes to clamp the discharging magnet coil currents that caused high voltage between relay contacts when the contacts were disconnected. You can try using those diodes in real life to protect the contacts of solid-state relay that you could be using. Solid-state relay uses electric switching and consist of semiconductors that can fail due to discharging currents from the two magnets coils in the circuit if the diodes are not used. However, connecting the diodes in the wrong polarity will draw high currents from the 1.5 V battery because a typical diode forward voltage is only 0.7 V. The battery might become hot and even explode.

Connecting the capacitors in parallel is equivalent to using a single capacitor of value equal to the sum of all capacitor capacitance values (Farad) that are connected in parallel.

The relay can be modelled as a coil connected in series with a resistor. However, I used a 555 timer model instead because it possessed Schmitt to trigger characteristics that exist in a typical relay. This could be due to relay magentic coil hysteresis properties. https://en.wikipedia.org/wiki/Magnetic_hysteresis

I found that using a comparator to model the relay would cause the oscillation to eventually decay. Initially, the relay is OFF and Sw1 switch is a short circuit. The four capacitors (C1, C2, C3 and C4) that supply power to relay begin to charge. At a certain point, the relay turns ON and the Sw1 switch becomes an open circuit. At this point, the four capacitors begin to discharge because their power supply is disconnected. This causes the relay to eventually turn OFF and thus restart the cycle. The second switch, Sw2, that is controlling power to the two electric magnets, switches ON and OFF at the same time as Sw1. The PSpice that I used did not have a three-terminal switch (that you will see in the next step photo). Thus I used two Sw2 switches instead (Sw2a and Sw2b) to model the relay switch.

The frequency of the oscillation is reciprocal to the sum of capacitor charging and discharging periods:

fo = 1 / (capacitor charging period + capacitor discharging period)

Calculating the charging and discharging period is beyond the scope of this article. However, the charging time can be increased by increase the capacitor and R resistor values. The discharging time can be increased by increased the capacitor values (is not influenced by R resistor that is disconnected during the discharging step of the cycle). Increasing R-value might prevent the relay from turning ON. This is why I used such a low 10-ohm resistor. Me using a 9 V battery to power a 12 relay was very risky. There was a small chance that my relay would not turn ON.

Step 2: Connect the Relay

Using a soldering iron is a very good idea to ensure reliable connections.

There were issues with my connections from 1.5 V battery to relay and the two magnets. The coil wire was very old and brittle. Thus was constantly breaking.

You can connect wires to 1.5 V battery using sticky tape, plasticine, or rubber band. Thus you might not need a 1.5 V battery harness.

Step 3: Make the Fish

I used very little metal wire for my magnets. I touched my 1.5 V battery and it seemed to be getting hot and losing power quickly. You should try using more turns because the magnet equivalent resistance was very low.