www.qrp4fun.de Modification for the L/C Meter IIB

In my opinion the one and only weak point of the L/C meter IIB by AADE is the voltage supply from a 9 V battery. To let not arise a misunderstandings: The battery is large enough for a lot of measurements. If one forgets its existence sometime and then exactly is the battery empty. A very well usable and effective modification presented Bruce Bedford, N1RX, at www.newenglandqrp.org/battery-saver under "Battery Saver for WM-2 Wattmeter". It can be used after a small change at the PCB of the L/C meter also here.
Circuit diagram
Circuit diagram with the modified switch-on the left The circiut diagram is the same one like Bruce's. Only the dimensioning was adapted to the existing electronic parts. R2 and C1 determine together the switch-on time of the L/C meter. Here there is approximately 60 seconds.
To restart of a measuring cycle the meter is to be switched-off briefly. Thus C1 can recharge again over the low-resistance resistor R1 and the FET VT1 is switched-on. After switching-on of the meter C1 discharges slowly over the high-resistance resistor R2. If the voltage falling below the gate threshold voltage the FET close and switch-off the voltage supply to the L/C meter.
In the switched-off condition R1 and R2 connected in parallel to the battery. That's why a current of 9 µA. The current is in order of magnitude of the self discharge current of the battery. The battery is nevertheless substantially longer usable by the battery saver. If switching-off the L/C meter was forgotten and if the switch remains thus in the switched-on condition, then no measurable current is more present after the battery saver switched-off (< 1 µA).
If you want to use the battery saver in a RF-device, the use of C2 and C3 are recommended. In the L/C meter there could be void.
Changes at the printed circuit board
Modifications for the switch PWR at the PCB Since the PCB of the L/C meter does not agree with the circuit diagram of the manual, there are a few changes necessary. Reason: The centre pin of the switch PWR is not connected to the positive terminal of the battery, but the break contact. Since however two toggles are joined in parallel, the second one can be used for the battery saver. With me at the available printed circuit board version are only two conductive strips to cut (markings 1 and 4) and an additional connection (marking 2) is required. Please control before cut the traces, in which traces cuts must be made. During the production it can have come to changes in the layout of the printed circuit board.
Battery saver at an universal PCB Installation of the add-on PCB
The additional circuit is inserted into the ground wire to the battery. In addition the black wire of the battery clip is to be soldered on to the add-on PCB (X2). An additional wire is soldered between the ground pad on the PCB and X1. The Pin X3 is soldered direct at the bottom side to the switch pins (marking 3). The small add-on PCB fits in inside between the battery box and housing wall.