How does the toaster know?

July 17, 2005 11:18 AM

How does a toaster's light-dark control work? Perhaps something clever with a bi-metallic strip... can't believe the toast's 'doneness' is actually being tested; but from experience it seems more sophisticated than just adjusting a timer. How do dem?

If I were designing a program to do this, it would poll a
bread_darkness_sensor, somehow.

posted by Rash to Technology (12 answers total)

they certainly used to be bimetallic strips. i haven't taken one apart in a long time, though - perhaps these days it's a little 555 timer circuit? and no doubt you can buy something that does measure colour, for some amazing price.
posted by andrew cooke at 11:29 AM on July 17, 2005

Best answer: Last year in my Engineering class we took apart several toasters to "reverse engineer" them. I learned nothing about reverse engineering but most of the toasters we took apart used a bimetallic strop like andrew cook mentioned. Basically it works thus:

When you push down the handle on a toaster a lock on a pivot swings into place and prevents the handle from moving upward. The handle is directly connected to a plastic wedge which, when lowered, spreads two copper rods enough to touch the sides and complete a circuit. As the bimetallic strip heats up it curves and once it curves enough it pushes the lock out of place and the handle is released, pushing up your bread and disconnecting the circuit.

The knob that controls your dark and light settings changes the distance the bimetallic strip has to travel to move the lock.

Since the bimetallic strip is heat sensitive, not time sensitive, I guess you could say it gives you a better "toast".
posted by cyphill at 11:55 AM on July 17, 2005

What I want to know is why, of all the toasters I've used, does a setting above 2 (of say 10) result in a cinder and the fire alarm going off even if the bread or whatever was frozen when I put it in?

What on earth are settings 3, 4, 5, 6, 7, 8, 9 and 10 used for?
posted by edd at 12:04 PM on July 17, 2005

I think you just need to get a better toaster, edd. I like my toast very light (just barely the slightest hint of light brown, almost more warmed bread than toast) and I use a setting of 3 or 4 out of 10 on my toaster to get that.
posted by DevilsAdvocate at 12:10 PM on July 17, 2005

It's still a timer issue in the end, the amoutn of time it takes to heat up the bimetallic strip to bend enough to do the trick.

I don't think that andrew cooke's suggestion/thought that you could get a color sensor for toast would work given that bread color varies so great white, sourdough, wheat, rye, pumpernickel, etc.
posted by FlamingBore at 12:18 PM on July 17, 2005

pah to you. rowenta toaster with photo sensor browning control. via.
posted by andrew cooke at 12:27 PM on July 17, 2005

another

[that's enough now. ed]
posted by andrew cooke at 12:28 PM on July 17, 2005

Howstuffworks has an entry on toasters. The forth page mentions the difference between a bi-metallic and timer based toaster.
The bi-metallic ones are not good if you need to toast say 4 waffles in a two slot toaster because you have to increase darkness for the second toasting to get ~ the same results as the first toasting.
posted by sailormouth at 12:51 PM on July 17, 2005

This reminds me of a humorous article about building a toaster: Electrical engineering vs. computer science.
posted by knave at 10:37 PM on July 17, 2005

edd, I might suggest the higher settings are for frozen bread and perhaps breads/bakery goods that are difficult to toast, like perhaps rye bread or bagel halfs.

I've never seen a toaster I can set higher than 3 and end up with a satisfactory toast (I like mine so light there's no brown, just crispyness, though...)
posted by shepd at 12:32 AM on July 18, 2005

I have an old, chrome, Sunbeam toaster that uses a slightly curved bar of metal with a small separate heating element underneath. I'm not sure if the bar is bi-metallic, but it is truly elegant. The element heats the bar, which expands and trips a spring-loaded catch, but the toast doesn't pop up at this point. Here's the clever part--the bar must cool and contract in order to trip a second catch which pops the toast. Why? So the second batch of toast is done the same as the first. The light-dark control simply adjusts the distance the bar must expand in order to trip the catches.
posted by weapons-grade pandemonium at 12:44 AM on July 18, 2005

its not really a timer, it is merely a heat sensor. when it reaches the required temp it shuts off. this is why the same setting will result in a shorter time and (imo) less overall doneness when used more than once in a row.
posted by sophist at 1:33 AM on July 18, 2005