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So for a school project where we have to make a lesson plan for kids to learn stuff out of "lame" experiments, I was assigned potato clock. I could not for the life of me find a simple 1.5 volt led clock, and I was not allowed to purchase something that is a kit or part of a kit. I ended up purchasing a simple rotary clock, instead.

The potato battery I rigged up puts out 1.60-1.80 volts so says my multimeter, but when I alligator clip it to the clock, nothing. I pulled a double a battery from a controller, and it works, but not the potato batterie (also tested the battery with the multimeter to see if the multimeter is broken, it came up right around 1.5 as well). Why will my potato battery not power the clock?

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  • \$\begingroup\$ Most quartz movement clocks should work, check the battery voltage with the clock connected. If it is very low, connect the meter in series and check now much current is being drawn. \$\endgroup\$ – Dean Franks Mar 14 '18 at 20:52
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    \$\begingroup\$ Look for an LCD clock, not LED. As others have noted, LED clocks use a lot of current, but LCD's use very little current by comparison. \$\endgroup\$ – Norm Mar 14 '18 at 21:39
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    \$\begingroup\$ There seems to be a whole lot of "science" sites on the internet that don't know the difference between an LED and an LCD. I found a metric crap load of sites recommending you get an LED clock to use with a potato battery. What's more, they seem to be ripping each other off. Many use nearly identical texts, that are all uniformly useless. Many also use the exact same photo of an LCD clock (and refer to it as an LED clock) and identical potatoes. This cries out for a properly done site that does it correcly and also correctly explains how it works (and why an LED clock won't.) \$\endgroup\$ – JRE Mar 14 '18 at 22:41
  • \$\begingroup\$ @JRE The same goes for a lot of "science" and "electronics" how-tos on Youtube and other sites. When they aren't outright faking their results, they're copying material from other sources -- often including other videos. \$\endgroup\$ – duskwuff Mar 14 '18 at 22:58
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    \$\begingroup\$ How many potatoes would it take to fry the chips in an LCD? \$\endgroup\$ – Transistor Mar 14 '18 at 23:18
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a simple 1.5 volt led clock

Well, there's your problem. You don't want an LED clock. You want an LCD clock. LEDs take (relatively speaking) a lot of current, and a potato battery simply won't supply it. As a check on this, go back to your potato/clock combo, and measure the voltage with the clock connected. Notice something? Your voltage is almost zero with a load on the potato.

I suggest you go on Youtube and search for potato clock. This will give you videos which provide a source for an appropriate clock.

If, as you say, you cannot buy such a clock, you are probably out of luck. About the only alternative is to find a clock, or perhaps a watch, which runs on a single alkaline battery. You can figure out which battery connections to connect to which electrodes you have and go from there.

But LED and mechanical clocks simply will not work with a single potato.

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The motor based clock (the square quartz clock module that takes a 1.5V AA cell and ticks once per second) requires short pulses (like 30ms) of fairly high current (few mA) more than you can get from a potato.

You could add a fairly large capacitor in parallel with the potato. It will charge up from the potato and supply those short current pulses to the motor. I think you'll need about 1000 uF. I just measured the coil on one, about 260 ohms so just under 6mA at 1.5V. dQ = I * dT = 6mA * 30ms = 180uC, so 1000 uF would give dV = dQ / C = 180mV ripple from the current pulses; good enough.

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This is probably because the clock requires more current than the potato battery can provide. Try putting more potatos in parralell, or find a clock that uses less current.

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    \$\begingroup\$ s/he doesn't need to use more potatoes, s/he can simply just reuse the same potato and put more terminals in parallel. \$\endgroup\$ – Harry Svensson Mar 15 '18 at 8:05
  • \$\begingroup\$ Thats indeed true, didn't think of that. \$\endgroup\$ – appmaker1358 Mar 15 '18 at 13:27
  • \$\begingroup\$ Some off-topic information, I checked out your profile and saw that you were interested in trains and games, if you haven't played Factorio yet, then you are in for a treat. There are trains and liquids and electricity and tons of other things. The liquids and electricity in the game has been developed to follow the real world physics, so you might find some inspiration for your game development. \$\endgroup\$ – Harry Svensson Mar 15 '18 at 15:55
  • \$\begingroup\$ @Harry Svensson, I am indeed interesstet in trains and games, but I like creating games and experimenting with physics, I don't play a lot of games actually. But thanks for the tip! \$\endgroup\$ – appmaker1358 Mar 15 '18 at 16:57
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The battery source series resistance must be much less than the load resistance at any time ( no pun intended ) like pulsing the solenoid wheel otherwise the voltage ratio declines according to the resistance ratio. You can measure resistance or ESR on a potato with short circuit current ( expected being less than max range of meter)

schematic

simulate this circuit – Schematic created using CircuitLab

The Short Circuit current must be >10x than the load current to maintain battery voltage within 10%.

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