Auto off for any circuit


Properly configured, your computer will go into sleep mode when left unattended for a long enough time. So will your cell phone, and just about every other piece of sufficiently complex electronics. Much simpler circuits, though, are left at the mercy of a SPST switch; if you forget to turn a flashlight off, it will be dead next time you need to use it. Wanting an auto-off circuit simple electronics, [Kyle] threw together this auto shutoff circuit.

The basic idea behind the cirucuit is to use a microcontroller as a timer controlling two transistors. When [Kyle]’s circuit is power cycled, the timer inside an AVR starts, making a pin high, and when the timer is up, making the pin low again. This pin feeds into a PNP transistor which is in turn connected to a NPN transistor, creating a very tiny auto off circuit for anything with an SPST switch.

[Kyle] says there are a few improvements to be made – using MOSFETS to handle higher currents and possibly using a smaller microntroller like an ATtiny 4/5/9/10 to shrink the circuit’s volume. It’s a great idea, bringing the idea of a flashlight with auto shutoff into reality.

Test your project’s mettle with a protected dummy load

For a power hungry project the supply is sometimes a pretty big unknown. Whether stapling together a few different power supplies to meet a current requirement, or designing a system from the ground up: a big power supply can be quite a dangerous thing. It helps to have some kind of a dummy load to really shake down the electronics and get an idea of how hot things get or test stability before trusting the supply to run your stuff. [Paulo Oliveira] has constructed just such a thing, a slick looking adjustable constant current load.

Following the popular LM324 circuit from [David Jones] at EEVblog [Paulo] decided to make use of the two spare op-amps to provide both a thermal overload and a cooling fan circuit. We have seen other tweaks to [David]’s circuit in the past but through some resistors and MOSFETs [Paulo] can now load up to 7A (limited by resistor wattage). We would have used a really crazy server vacuum fan to make it genuinely frightening to push heavier loads. Thanks [Paulo]!

Automatic speaker control via TV

[Jon] wanted his speakers to come on and off along with his TV. The speaker heats up if left on so he didn’t want to do that. But killing the power also resets the volume level (this is an old set of PC speakers and the remote is wired, not IR) so using one of those switched power strips was out as well. He thought a bit about trying to use the power LED on the TV to build his own circuit when it dawned on him. It’s possible to monitor the USB port on the TV and use it to switch on the speakers.

The circuit above uses a couple of opto-isolators to protect both the television and the speakers. The 5V line from the USB port on the back of the TV is monitored by an XNOR gate (which helps to filter out some of the toggling at power-on). When that gate latches it activates a 555 timer which in turn fires up the speakers. Presumable this happens when power is cut as well, but we’ll let you work through the circuit logic yourself.

Constant current dummy load

Inspired by a design he saw on the EEVblog, [George Graves] put together this constant current dummy load.  You might need on of these if you’re testing power supplies or batteries. They pull a constant current regardless of the voltage of the supply. [George’s] version extends the range of the original a little bit by running the op-amp at 8 volts. He says that everything runs fine at 1 amp. He tried 2 amps but things got hot pretty quickly. What we really like though, is he took fantastic pictures. Sometimes even simple things can catch our attention with the right pictures!

Build an induction heater and become a metalsmith

If you’ve ever wanted to forge, cast, or smelt metal, this project is right up your alley. It’s a 30 kVA induction heater built by [bwang] over on Instructables. It gets hot enough to melt and forge steel, iron, and aluminum.

An induction heater operates by surrounding the object to be heated with a coil carrying high frequency AC current. Basically, the entire setup acts like a huge transformer with a shorted secondary. To get these currents into a workpiece, [bwang] used a TL494 PWM controller as an oscillator. The output of the TL494 is filtered and amplified a few times to generate a huge amount of AC current.

Larger versions of [bwang]’s induction heater are found in foundries and forges all across the land; even though this small version sucks down 50 A out of a dryer or stove outlet, induction heating is very efficient. We’re actually wondering why we don’t see many home blacksmiths using induction heating, so we’ll leave that for our readers to discuss in the comments.

[sessions] reminded us of this induction heater from a few years ago. A little smaller, but still usable.

Auto power circuit for an arcade machine

Some of the pinball machines which [Jeri Ellsworth] has restored have ended up in the break room at her work. We’re sure her coworkers are thankful for this, but sometimes they forget to turn off the power to the machines, and letting them run constantly means more frequent servicing will be necessary. She set out to fix the situation by building a circuit that will automatically power the machines.

We think the solution adds some much needed functionality. Instead of hunting for the power switch, you can now power the machine up by hitting the left flipper, and it will automatically shut off after about five minutes of not having that flipper button pressed. For this she grabbed a 555 timer chip and built a circuit to control the relay switching the mains power.

She added a magnet and reed switch to the left flipper switch assembly to control her add-on circuit. It connects to the base of a PNP transistor which controls a resistor network and capacitor. This part of the circuit (seen to the left of the 555 in the schematic) allows the timer to be re-triggered. That is, every time you press the flipper the 555 will reset the timer. Don’t miss the demo she filmed after the break.

Continue reading “Auto power circuit for an arcade machine”

USB killswitch turns off your home entertainment bling automatically

Finally, the USB port on the back of your television can be tapped for something useful. [Don] is using this add-on device to automatically cut the power to his Ambilight clone. Initially, he got tired of unplugging the power adapter each time he shut off the television, so he added a switch. But laziness overcame him and he decided he needed an automatic method. After probing around on the connections available, he established that the serial interface (normally used for servicing the device) was not of any use, but the USB port is. He measured the voltage of the power bus to be 5V when the TV is on, and 0.15V when it is off. He whipped up the circuit you see above which uses the USB connection to trigger a relay, connecting power to his Ambilight clone when the television comes on, and disconnecting it when the set is switched off.

Our dream has always been an XBMC capable device that can Velcro to the back of a TV, and be powered from that USB port. Unfortunately the Beagle Board hasn’t yet made it to a stable level when running XBMC. Our next hope is the AppleTV 2, which can run XBMC but would require some hacking to get it working off of the USB port, raising concerns about how much current it would draw at 5V.