I want to attach a generator to a door handle and have the output charge a battery. My system needs to provide about 3 Watts. A similar question was asked here and I referenced it to get an idea of what kind of force, torque, power, and energy are needed to turn the door handle. These values, however, were calculated without the generator attached to the door handle. My question is, how would attaching the generator to the door handle affect these values? I know that the generator will also require some torque to turn it. What I'm confused on is if I should just connect the generator directly to the door handle or use gear ratios to figure out some gear system? The gear system would be to increase the angular velocity of the generator based on the gear ratio and the angular velocity of the door handle. Generators that I have looked at usually have some rating where they say what voltage and current are produced based on the rpm, so I figured I could get these rpms with gears, but gear ratios also relate the two torques.

I know there is more than one question in here, but if you could help me with understanding any of this I would really appreciate this. Even if this isn't quite feasible, what would be the major constraints?

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    \$\begingroup\$ 3 watts continuously, like powering a raspberry pi, or 3 watts for an instant, like powering a transmitter for a burglar alarm or door chime? \$\endgroup\$
    – tomnexus
    Commented Jan 29, 2015 at 3:45
  • \$\begingroup\$ Please describe what you are planning to do with the charged battery. If you can't do that, please describe your expectations about how long it will take to charge the battery, and how much power will be removed from it. My biggest concern is that there is not going to be enough mechanical power applied to the door handle to meet the needs of whatever you are doing. Oh, and the battery size in Watt hours or mAh, plus nominal voltage. \$\endgroup\$
    – user57037
    Commented Jan 29, 2015 at 4:49
  • \$\begingroup\$ The 3 Watts that I mentioned was an earlier estimate. This battery should be able to power a pic 24 continuously at 0.63 mW, should be able to keep a fingerprint reader on continuously at 429 mW, and keep a wifi module in standby mode continuously at 3.96 mW. I am also considering attaching a dc motor to the deadbolt latch and driving it with an h-bridge. This would also be powered by the battery at 1 W, but this isn't continuous and would last for about 0.5s. As far as more specifics with the battery goes, I'm not really sure yet. I want it to be able to do what I mentioned in this answer. \$\endgroup\$
    – Jucesanc
    Commented Jan 29, 2015 at 5:24
  • \$\begingroup\$ Thank you for the answers by the way. Something else that I'm confused on is how to calculate the torques required for the gear ratios. I know this is a non-electrical engineering topic, but if anyone has good resources for this that'd be great! \$\endgroup\$
    – Jucesanc
    Commented Jan 29, 2015 at 5:29
  • \$\begingroup\$ If the fingerprint reader is really 430 mW and needs to be on continuously, meaning 24/7, that is going to kill you. Can it not be put to sleep until some event happens? Maybe you can use an accelerometer to wake it up when you tap it? (BMA 222). As far as how to harvest power from a door handle, I think maybe a permanent magnet motor. Could be linear or rotary, whatever is convenient. Instead of mechanical gearing, I think you can use DC-DC conversion to harvest the maximum power without making the action feel heavy. \$\endgroup\$
    – user57037
    Commented Jan 29, 2015 at 6:35

3 Answers 3


This battery should be able to
power a pic 24 continuously at 0.63 mW, should be able to
keep a fingerprint reader on continuously at 429 mW, and
keep a wifi module in standby mode continuously at 3.96 mW.
I am also considering attaching a dc motor to the deadbolt latch and driving it with an h-bridge. This would also be powered by the battery at 1 W, but this isn't continuous and would last for about 0.5s.

You say you want a continuous mean power output of
0.63 + 429 + 3.96 mW + 1W very occasionally
= 429mW + irrelevant.

In a day that's about 10 Watt.hours

So a user inputting 3 Watts continuously via a door handle would need to do so
10/3 = 3 + hours per day.
Or 30 W for 20 minutes - that's a significant exercise level.
Or 300 Watts for 2 minutes - most people could not do that.

ie you can easily see with such simple calculations that what you have asked for is unrealistic AND that you do not need most or the power you requested. It seems extremely unlikely that you need a continuously powered card reader or that it would need siuch a high poer level in standby mode.

Try again.
5 mW to 10 mW average should be enough.
Over 24 hours, 10 mW =~ 0.25 W.h
0.24 W for 1 hour
2.4 W for 6 minutes
24 W for 36 seconds.
If you can obtain that in 10 sessions it's 24W x 3.6 seconds x 10
Or 2.4W x 3.6 seconds x 100

The last figure sounds like what you might easily be able to obtain from opening a door without it bein too too obvious. In practice you'd want several times more to allowfor inefficiencies and practicalities.

Is 100 operations a day too many - that will depend on application.
But the above gives you a basis for a realistic calculation using assumptions that suit you.

Once you have refined the spec we can talk about power generation.

At 100% efficiency

Watt.seconds ~= kg_force x metres travelled x 10.

This could be derived from opening a door, pulling on a lever or eg standing on a step (that moves under applied weight to power an alternator).

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    \$\begingroup\$ I think what is needed is as follows: A single turn of the handle needs to replace all of the energy that was expended in reading the fingerprint plus a little extra for the Wifi. That is what it actually comes down to. So the OP has to figure out how to sleep the reader in between sessions. \$\endgroup\$
    – user57037
    Commented Jan 29, 2015 at 6:38
  • \$\begingroup\$ @Mkeith Sounds good. 500mW say x 1 mins say = 30 Joule = 3 kg.m @ 100%. Opening a door could do this nicely. | = 30 kg x 100mm or 60 kg x 50mm @ 100%. Weight on step etc. \$\endgroup\$
    – Russell McMahon
    Commented Jan 29, 2015 at 7:50
  • \$\begingroup\$ I redid my calculations. The components that I need are a pic24, wifi module, h-bridge, and a fingerprint reader. For wifi I have 2 power modes: sending and saving. For the system I have 3 modes: burst, UI, and idle. Different modules are used in the 3 modes and I calculated how much power is required in each mode. Then I multiplied each of those by the time each mode is on for. The sum of these gave me gave me the total energy needed per day; 39.02kJ = 10.84Wh. My calculations matched up to Russel's. The fingerprint reader doesn't have a sleep mode, but maybe I could have a switch on it? \$\endgroup\$
    – Jucesanc
    Commented Jan 29, 2015 at 9:39
  • 1
    \$\begingroup\$ I would use a very low power small cheap micro to sleep with one eye open, waiting for a user input (if this requires an extra sensor near the fingerprint reader to detect a finger then so be it), and then power up the rest of the system only when required. Power harvesting from the door handle is likely less useful than something like a small solar panel on the unit or using the door opening (much larger movement, much more power over a longer period) \$\endgroup\$
    – John U
    Commented Jan 29, 2015 at 14:52

Consider getting power from the opening of the door instead. It's a much longer lever and the user can apply more force. For example, you could have a weight on a cable that's lifted on opening, then slowly descends powering a generator. You could even have it descend very slowly like a clock weight, although efficiently generating electricity from that may be tricky. Whatever you do charge/discharge inefficiency + self-discharge of a battery is going to consume quite a bit of power.

You can also do much better on the power consumption. Capacitative sensing can be done with very low power consumption. If the mounting plate for the fingerprint reader is a sensor then you can have it woken up on demand. Don't forget to add a couple of indicator LEDs to confirm that the reader is actually working if you do this, so you can debug being locked out.

(I still think it's much easier and more reliable just to have a battery or mains connection; have a look at e.g. Lockitron.)


Energy harvesting is for micropower devices, not wifi, actuators and continuous drain devices.

Look for a hand cranked USB charger to get an idea of a practical, cheap generator, and read the reviews to see how much / little power it generates.

If you can't get external power, your best bet is a primary battery, or solar cells for charging.

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    \$\begingroup\$ FYI only: Small plastic geared hand crank chargers may get about 1 Watt if enthusiastically operated and after about 5 minutes of use your hand will need a holiday. A well designed hand crank charger will produce 5 Watts "all day long" and you'd be liable to die of boredom rather than exertion. 10 Watts is getting a bit enthusiastic but is bearable. More is possible but gets annoying. For pedal power 50W is doable over 1 hour and registers as lightish but noticeable exercise. 100W is annoying.More gets hard quicklyish for average fitness people. \$\endgroup\$
    – Russell McMahon
    Commented Jan 29, 2015 at 12:28
  • \$\begingroup\$ @RussellMcMahon Thanks, that's better input than I could find quickly online. 5 or 10 W isn't bad, but unless each person is asked to crank the door for a few minutes, it won't add up. \$\endgroup\$
    – tomnexus
    Commented Jan 30, 2015 at 6:19
  • \$\begingroup\$ That's a much better handcrank than you'll usually found sold. Essentially a "smart drive" washing machine motor - effectively a lowish speed BLDCM that will directly agitate and spin a bowl full of clothes + water - with a hand crank added. You can make similar but the Fisher & Paykel machines are a good cheap way of doing such things. \$\endgroup\$
    – Russell McMahon
    Commented Jan 30, 2015 at 7:45

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