When connecting loudspeakers to the device, one should observe the polarity of the connectors if there is only one electrolytic capacitor in each loudspeaker box instead of two of them tied back-to-back (as is depicted in the block diagram). The reason is that if the electrolytic capacitor is tied to the inverted DC voltage, its high bleeding current would be able to deactivate the squelch circuit on its own without pressing the "VOX on" key. And worse, if left in such erroneous state long enough it may even explode. So pay some attention to it.

Current consumption of the circuit in the passive state is approximately 8mA which means that a set of alkaline batteries should be sufficient for supplying it non-stop for over a week. The actual lifespan of the batteries naturally depends on how frequent the circuit gets activated. When powering the monitor by an AC/DC adapter, there is no need to pull out the batteries as they can neither be damaged thanks to the protective diode D5, nor can they be depleted as voltage coming from the AC/DC adapter is higher than the DC voltage batteries produce. Just remember to pull out the batteries once the kid grows older, as if left in place they might leak over years and permanently damage electronic components. Diode D6 protects the whole circuit from connecting the external power supply in reverse.