Walkie-talkies (also often referred to as handheld or PMR, Personal Mobile Radio) can be bought at low prices even from department retailers, and they are able to be operated with no licence in many international vicinitys. Considering the low value, such a set can be very appropriate to be used as a wireless baby reveal, with the addition of a few external components. These are linked to the jack sockets for an exterior loudspeaker/microphone and an external PTT (Push-To-Talk) change, which would possibly be incessantly discovered on these tools.
Project Image :
Wireless Baby Monitor Project Image
The walkie-talkie with the additional digitals and microphone is positioned within the baby’s room. When the PTT switch on the opposite walkie-talkie is actuated for approximately a 2d the ‘baby’ walkie-talkie produces a sequence of tones, which the exterior electronics can observe. This then prompts its own PTT swap for roughly 5 2ds, so it swapes over to transmit. During this time the different instrument can hear what the exterior microphone picks up.
Circuit Diagram :
Figure 1-Wireless Baby Monitor Circuit Diagram
Figure 1 shows the circuit that the writer designed for this. It has been designed namely for a Tevion three000 PMR bought a while ago with the help of Aldi. This kind of PMR has a combined jack socket that embraces all the required connections.
The voltage present on the PTT connector is used to generate the provision voltage for the circuit by the use of R3, D1 and C1/C2. When the loud-speaker output presents a sequence of tones (when the PTT change on the opposite walkie-talkie is held down), it lead tos T1 to behavior. This also turns on T2 and T3, so that the external microphone is related to floor. The ensuing present that f lows through the microphone must be sufficient to prompt the PTT circuit within the walkie-talkie, inflicting it to transmit. If the external microphone doesn’t draw sufficient present, a resistor (R8) will have to be linked in parallel. Some experimentation with the worth of this resistor may be required. If you are looking to have to make use of the inner microphone then R8 should get replaced with a wire hyperlink.
Circuit diagram :
Figure 2-Wireless Baby Monitor Circuit Diagram
When the walkie-talkie changees to transmit the built-in amplifier ceases producing a sign and T1 flips off. However, due to the fact electrolytic capacitor C3 has been charged up in the interim, transistors T2 and T3 will preserve habitsing for a couple of seconds until C3 has been virtually discharged via R4. In the Elektor labs a more practical version with the identical functionality (Figure 2) has been designed for use with a more cost effective PMR set that is additionally received from Conrad Electronics (PMR Pocket Comm Active Pair, order number 930444). These walkie-talkies have separate jack sockets for the LS/Mic and PTT joinions.
When there is a call a series of tones is produced that is used to activate T1 by means of R3. T1 then prompts the PTT perform and the microphone amplifier is turned on. How-ever, it ’s not just the audio sign that's used, but additionally the DC offset produced when the interior output stage is turned on. Both the interior in addition to external loudspeaker are pushed by means of an output capacitor of a hundred µF. When there's a call it charges up by means of R3 and the base-emitter junction of T1. If the walkie-talkie is known as frequently there could be a risk that the output capacitor would remain charged and the DC offset of the audio signal would now not be sufficient to activate T1. To prevent this, D1 is hooked up in reverse across the base-emitter junction of T1, pro-viding a discharge path for the output capacitor.
To maintain the circuit energetic for a minimum period of time the microphone voltage is used to supply an additional base present. This is finished by means of charging C1 by approach of R1. When the transmitter is fliped off the microphone and R2/ D1 provide a discharge path for the capacitor. C2 make certain thats that the circuit won’t react to spikes resulting from interference. As can also be seen from the 2d circuit diagram, use is manufactured from two joinors, a 2.5 mm jack plug for an exterior headset and a 3.5 mm plug for the PTT perform. These joinors are explicit to the walkie-talkies we used right here. With other types of walkie-talkie you must first take a seem to be at the joinion important points of the connectors sooner than you connect the circuit up.
When the circuit is used as a child monitor you should test that the microphone you’re the use of can decide up all the sounds. In our case the microphone didn’t seem like very sensitive. The microphone amplifier has almost definitely been designed for a voice that's close to the PMR unit. When used as a child screen the microphone must subsequently be placed as with reference to the infant as that that you can suppose of.
Author : Wolfgang Papke - Ton Giesberts
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