Neil Robinson
Registered
Intrigued by this topic, especially why a 5V circuit needed a relatively high input voltage to work and with time on my hands I decided to investigate further. I ordered a dimmer from China for the princely sum of £3.77 inclusive of shipping that arrived yesterday.
Inside it looks like this.
The 5V regulated supply for the control circuit is provided by the first four components from the left.
These are a 470 Ohm resistor, a 1N4001 diode, a 10uF capacitor and a 78L05 5V regulator.
In my opinion the resistor is a good idea with supply voltages of 12V or more but is responsible for an excessive voltage drop with lower supply voltages. Without the resistor experimental results suggested that to overcome the internal voltage drops in the diode and regulator a supply of an absolute minimum of 7.2V would be required. I had a reasonably charged battery with an output of 8.8V so I used it to check. With the resistor in place the unit wouldn't work and the output of the 5V regulator was only 3.8V with a 2.8V drop across the resistor. With the resistor shorted out (and therefore an extra 2.8V available to the regulator) the regulator output was 5.05V and the unit worked well.
From this I concluded that the unit would work reliably with a nominal 8.4V battery provided the resistor was shorted out. In my opinion an effective way of doing this is to solder a wire on the underside of the board as shown below.
Naturally for supplies of 12V or more this modification isn't required.
Regarding range I suspect the state of the battery in the keyfob is relevant. I also suspect that, for the price, the battery supplied may not always be the highest spec. and freshest example of its type.
Inside it looks like this.
The 5V regulated supply for the control circuit is provided by the first four components from the left.
These are a 470 Ohm resistor, a 1N4001 diode, a 10uF capacitor and a 78L05 5V regulator.
In my opinion the resistor is a good idea with supply voltages of 12V or more but is responsible for an excessive voltage drop with lower supply voltages. Without the resistor experimental results suggested that to overcome the internal voltage drops in the diode and regulator a supply of an absolute minimum of 7.2V would be required. I had a reasonably charged battery with an output of 8.8V so I used it to check. With the resistor in place the unit wouldn't work and the output of the 5V regulator was only 3.8V with a 2.8V drop across the resistor. With the resistor shorted out (and therefore an extra 2.8V available to the regulator) the regulator output was 5.05V and the unit worked well.
From this I concluded that the unit would work reliably with a nominal 8.4V battery provided the resistor was shorted out. In my opinion an effective way of doing this is to solder a wire on the underside of the board as shown below.
Naturally for supplies of 12V or more this modification isn't required.
Regarding range I suspect the state of the battery in the keyfob is relevant. I also suspect that, for the price, the battery supplied may not always be the highest spec. and freshest example of its type.