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Ben takes an off-the shelf baby monitor and creates adaptive headphones that switch between a music device such as an iPod or phone and noise from a secondary input such as a crying baby. He does so using integrated circuits, discrete components built into the case. Bit by the bug?
Ben takes apart a baby monitor to see if he can build the circuitry inside of it. Inside the baby monitor is a radio module, a glop top integrated circuit that runs everything, power regulator, crystal that runs the circuitry, some caps, an antenna, and an IC that might be some sort of memory or identification chip. It might be possible to use the battery that powers the baby monitor to also power the switching circuit.
The speaker could also be removed entirely and turned into a headphone jack. In order to see if the amplification going to the speaker needs to be knocked down a bit to prevent hearing damage, Ben hooks it up to a battery to see how much current it draws. It’s drawing about 50 milliamps which isn’t too bad so Ben continues testing with Felix simulating baby sounds.
For this project Ben uses the ADG 436 integrated circuit, a two channel switching circuit, so they can use logic on or off to switch from one channel to another. The outputs are hooked up to headphones and a pair of Nintendo Game Boys that act as inputs. There are pull down resistors so that both of the inputs are set to low.
Ben looks for a way to combine the switching circuit with the baby monitor. The switching circuit has been tested to work down to about 1.8 volts. He then probes around to see where all the voltages are so that he can power the switching circuit. He finds a boost regulator used to boost the lower level of the batteries up to a stable level of 3.2 volts for device operation. He also finds that the system voltage can be found on many test pads around the unit as well as the positive terminals of several of the electrolytic capacitors. He then uses his bench power supply to simulate the batteries losing charge and discovers that the system kept working down to around 2.3 volt battery level.
Once Ben is able to get the baby monitor to switch correctly between the inputs he goes to work getting it to switch automatically. By default the switching circuit plays the music input channel. Ben uses a 555 integrated circuit timer and some capacitors to get it to switch over when noise is detected on the other end and leave it switched for a minimum amount of time. Attaching some small surface-mount capacitors around it will allow the 555 to be triggered by the LED indicator and then it will keep the signal line high for four seconds so that it can switch back to music if an insignificant sound is detected.
Ben wires up a 555 timer in monstable mode, aka one shot mode. When the indicator LEDs are turned on what’s actually happening is that they’re being sourced with 2.6 volts and then the microcontroller pulls the other side of them low allowing current to flow through the resistor. Normally the LED is high but when it’s active it’s low. The LED is hooked up to the input or the trigger of the 555 so whenever there is a falling edge on the 555 the one-shot triggers for a specific amount of time. This amount of time is controlled by a specific resistor and a capacitor.