I'm trying to see if it's possible to use my landscaping lighting (low voltage lighting -12v) to power an additional appliance that is 9 volts. Can I "tap into" the wiring with some type of resistor and simply plug into the appliance? The appliance uses 4 C batteries or a 9v adapter. I just want to hard wire it to my landscape lighting instead. My transformer could handle the additional wattage draw.
I'm really new at this so pardon the ignorance. Thanks.
You need a bridge rectifier, a smoothing capacitor and a low-dropout 9 V regulator on a small heat sink.
The bridge rectifier can be the cheapest you can get. It will probably be rated at 1 A and 50 V or more, but larger ratings don't matter.
Your smoothing capacitor should be 2200 uF or more, and 25 V, or more.
For the regulator, https://www.fairchildsemi.com/ds/KA/KA78R09C.pdf would do.
You might get away without a heat sink. If it gets too hot it will shut down. If it runs to hot to touch, add a heat sink.
If you put that together, it doesn't matter whether the 12 V is AC or DC, you will get 9 V out. The other advantage of the bridge rectifier is that if the 12 V is DC, it won't matter which way round you connect the input of the rectifier to the 12 V. The output side is important to connect correctly.
Being the device is 6 - 9 volt input and a rectified and filtered 12 AC source will yield around 16 -17 VDC a normal voltage regulator IC like a LM317 would work just fine. Also being battery powered would suggest the actual load power is very low and the regulator IC may not even need a heat sink.
Likely a 25 volt 470 uF capacitor on the input side of the IC and a 10 uF on its output side would be more than enough for the few tens of Ma's the pest repeller draws.
If the battery input can be tapped into a LM7805 with a pair of diodes on the common side to raise it another volt or so would put its output in the same range as what the four batteries nominal voltage is also.
Or go online to one of the countless electronics supply companies and just order a LM7809 9 volt regulator IC and the pair of capacitors for a few dollars.