For the Hamburger Unterwelten e.V. , we needed a digital voice recorder (which is comercially available for about 15 Euro) that emits sound from time to time, preferably unattended. However, the chosen device has to be started by pressing a button one, then it talks for 20 seconds and falls silent again -- until the next press of the start button.
This small (and simple, thanks to the 555 timer IC) circuit emulates a press on the button every 42 seconds, keeping it "pressed" for about 22 seconds and then releasing it for 20 seconds.
The voice recorder runs at 4.5V DC and consumes about 60 mA, so this impulse circuit is also operated on 4.5V DC. The NE555 is specified at a minimum operating voltage of 4.5V, and it actually does run reliably this way. Power is provided by a wall wart providing 200mW at adjustable voltages. The inner pin of the power supply plug must be +4.5V, the outer ring of the connector must be GND.
The MOSFET output pins at JP1 are connected to the "PLAY" connector of the digital recorder module, so that GND of the module is at JP1.2, and the positive voltage measured on the module's PLAY connector is connected to JP1.1 of the timer circuit.
For production use, the two PCBs (timer and voice module) are put into a plastic case providing an internal speaker, connectors for the power supply and sound output (3.5mm stereo jack), and a switch to choose between internal speaker (marked "Spk") and external sound output jack (marked "Ext").
Usually, the Quasselstrippe starts talking as soon as power is supplied to the box. If it does not, the user can either wait for the current timer cycle to complete, or remove and reapply the power connection.
Inside the case, the "RECORD" function button, a microphone, and the control LED of the voice recorder module can be found and used if the recorded message is to be changed.
Part number YA-B01.01 has been assigned to this device.
|JP1||power supply +4.5V 100mA|
|JP2||voice recorder play button|