Some motors can draw more current than a USB port is designed to supply to peripheral devices. So, for motor control, we typically use a battery pack to supply the motor current. In this activity, you will:
(1) Standard servo - 3 V
(1) AAA cell battery pack
(2) New alkaline 1.5 V AAA batteries
When a battery pack is plugged into the Edge I/O adapter, the voltage/current it can supply is available through the left bus strip’s (+) and (-) columns.
Make sure to remove the batteries from the battery pack when it’s not in use. Otherwise, the circuit will drain the batteries.
JST is an abbreviation for Japan Solderless Terminal, a standard established by JST Mfg. Co. Small, white plastic connectors like the ones you see on the micro:bit and the Edge Adapter are commonly used inside products and appliances. We don’t normally see them since they are on printed circuit boards (PCBs) that are built into the product. The black fiberglass micro:bit and Edge I/O Adapter boards are examples of PCBs.
When your battery holder has alkaline batteries and measures 2.8 V or less, it means the batteries have lost much of their charge. This can lead to sluggish or nonexistent servo motor motion. So, this test is important to make sure the batteries are not too low for servo control.
A single AAA alkaline battery is considered to be a 1.5 V supply when used in rough calculations. Two alkaline batteries connected in series (end-to-end) have their voltages added. Since the AAA battery holder connects the two AAA batteries in series for you, it is considered to be a 3 V supply.
When alkaline batteries are new, they typically measure around 1.6 V. As they supply current to circuits, devices, or motors, they lose their charge. Charge is the accumulation of electrons, and a battery’s charge decays as it supplies current (the flow of electrons). By the time the alkaline batteries in series reach 2.8 V, the batteries are considered “dead” for micro:bit servo control.
Although alkaline batteries have a voltage charge relationship where voltage decays with charge, there are also different kinds of batteries that have different voltage to charge relationships. For example, lead-acid batteries found in older model cars have to be tested by a machine that applies a load and measures how much current the battery supplies. Another example, nickel-metal hydride (NiMH) rechargeable batteries have a much smaller change in voltage to indicate their charge level.
If you have some dead AAA batteries:
When a USB is connected to the micro:bit, the Edge Connector connects the USB supply’s GND voltage to the left bus strip’s (-) rail. It also connects the regulated 3.3 V from the micro:bit to the 3.3 V pin. When the battery is connected its (-) terminal is also connected to the left bus strip’s (-) rail. This is called a “common ground”. In this case, ground is common to three systems: the USB 5 V supply, the micro:bit’s 3.3 V supply, and the battery pack’s 3 V supply.
THE DIFFERENT POSITIVE SUPPLY VOLTAGES SHOULD NOT BE CONNECTED IN COMMON. That is why the battery pack’s Vbat supply is only available from the left bus strip’s (+) rail. The micro:bit’s 3.3 V supply is available from the center and right bus strip’s (+) rails, which are connected to the Edge I/O Adapter’s 3V connection with jumper wires you added in the Set Power for Circuits activity’s Parts & Circuit page. Since the battery pack’s voltage might be 3.2 V when the batteries are new and 2.8 V when they are almost dead, it is called an unregulated supply. In contrast, the micro:bit module’s 3.3 V supply is considered a regulated supply. For example, with the USB cable connected to the micro:bit, it might supply something in the 4.5 to 5.5 V range to the micro:bit. But, the micro:bit’s built-in voltage regulator would still supply 3.3 V at the 3V pin. The job of a voltage regulator is to supply unchanging or “regulated” voltage.
Let’s compare the unregulated battery voltage to the regulated micro:bit voltage. To do this you will, probe the center bus strip’s (+) and (-) rails. In other words:
The voltage you just measured is the voltage regulated by the micro:bit, so your voltage measurement should be closer to 3.3 V. Again, while the unregulated battery voltage will probably be somewhere in the 2.8 to 3.2 V range, when the micro:bit is connected to USB, the supply from the Edge Adapter’s 3V pin should be very close to 3.3 V.
Links
[1] https://learn.parallax.com/sites/default/files/content/Python/breadboard/hex/measure_volts.hex
[2] https://python.microbit.org/v/2
[3] https://cyberscope.parallax.com/