To begin today’s post, we will make a slight rectification to our earlier posts. In prior posts, we stated that we are going to use a 3.7V Lithium-polymer (LiPo) battery. However, more specifically, we are using a 3.7V Lithium-ion (Li-ion) battery. While there are some differences between the two types of batteries, since they are both lithium-based batteries, they utilize the same chemistry for the most part.
Li-ion batteries are the ones that are found in most cellular phones. We found that these batteries are more favorable to use in our project than their LiPo counterparts. Whereas LiPo batteries suffer from a shorter lifespan, Li-ion batteries are more efficient and have higher power density, meaning they contain a large amount of energy in a small package. Furthermore, Li-ion batteries cost less than LiPo batteries. Even though Li-ion batteries are “potentially dangerous”, we won’t be driving heavy loads, and we will be testing in an open environment, rather than a confined space. A comparison of the two batteries is best summarized in the image below:
The second part of today’s post concerns the powering of our circuit. In a post we made a few weeks ago, we mentioned that the 3.7V battery would be good enough to power the Arduino MEGA. According to this website, at 3.7V, the MEGA’s clock speed is approximately 16MHz, while at 5V, the clock speed is the complete 20MHz. So, running the MEGA at 3.7V, would run it at 80% of its full clock speed. We initially tested whether or not this would work and power the Arduino. It did work with just the Arduino and the RFID. However, we found that it did not work with the Arduino, RFID, LED, ultrasonic sensor, and motor driver all connected because while the RFID runs at 3.3V, the LED and ultrasonic sensor both run at 5V. Thus, we looked for a way to resolve this issue.
Our solution came in the form of a boost converter, which increase voltage at the cost of decreasing current. We chose the MT3608 boost converter, and after testing, we found it was able to boost the 3.7V battery voltage up to the desired 5V (with a maximum of 28V) to power the circuit: the Arduino, RFID, LED, ultrasonic sensor, and motor driver. We chose this particular board for several reasons:
- the boosted voltage can be easily adjusted with the on-board potentiometer;
- it’s easy to integrate into our circuit;
- it contains a protection circuit to avoid over-voltage charging;
- it possesses lower operating voltage (i.e. minimum voltage required to power it);
- it could be delivered the quickest as it was from Amazon Prime, and it was a pack of 10.
The other option we had was to use OPAMPs, but we couldn’t find an OPAMP with low enough operating voltage (i.e. minimum voltage required to power it), unless it was a surface-mount chip (SMT). For example, if got an OPAMP that required 9V to operate, we would need a 9V battery, and the mini-battery charger we will be using can only charge up to 5V. The 9V battery would have to be recharged with the large Lithium battery charger, which cannot fit into the pod car. This would defeat the purpose of using a more portable charger and the implementation of induction charging. An image of the boost converter is shown below, as well as the Amazon purchase link with all of the product details:
Source: https://www.amazon.com/gp/product/B07FYXSWJ8/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
On a final note, Dr. Furman listed some places for fabrication during class. Although this was directed more towards the track teams, this was helpful in case we need to manufacture something small: Allen Steel in Redwood City and Master Metal and Vender Bender on De La Cruz Blvd in San Jose.
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