1. What happens to a coin as it rolls through a vending machine?
When you put a coin into a vending machine, it rolls along a path inside the machine. According to patent descriptions, traditional coin identification devices placed coil sensors facing each other on both sides of this path. They compared the output waveform to standard values based on the coin's material, diameter, and thickness to determine if it was genuine [3]. A coil is a wire wound into loops. When alternating current flows through it, a magnetic field is created around it. When a metal coin enters this field, an eddy current (a swirling electric current) is generated inside the coin, which changes how easily electricity flows through the coil. Reading this change is how eddy current sensors work. Papers on coin classification state that eddy current sensors are the mainstream method in common vending machines [1]. However, this is just one common method, and not all machines necessarily use the same system. So, what can be learned from these electrical changes?
2. How do vending machines measure a coin's thickness and material using a magnetic field?
In one patent, an alternating magnetic field from a transmitting coil creates eddy currents in the passing coin. A receiving coil then measures the secondary magnetic field. The resulting waveform takes the shape of two peaks and one valley [2]. According to the patent, the value of the valley is less affected by the coin's thickness and is better at capturing differences in conductivity (how easily electricity passes through, which relates to the material). On the other hand, the value of the peaks changes due to both thickness and conductivity [2]. If the material can be estimated from the valley, the thickness can then be estimated by looking at the peak values while accounting for the material's effect. Combining the peak and valley values to measure thickness and material separately is the concept behind this patent. This is just one method described in a patent; it cannot be said that every manufacturer's machine uses this specific configuration. Can a coin made similar in size and weight be identified this way?
3. How do machines deal with counterfeit coins made by shaving 500-won coins?
Traditional methods captured coin features on average. Therefore, patents state that they could not completely eliminate counterfeit coins made by shaving one or both sides of a South Korean 500-won coin to make its weight and thickness close to that of a 500-yen coin [3]. This patent proposed using two coil sensors with diameters smaller than the raised patterns on the coin's front and back, processing their outputs separately. Smaller coils make it easier for the output to reflect differences in shaved areas. Additionally, a sensor to check the shape of the coin’s edge was added to find traces of shaving [3]. This is a proposal in the patent, and not all machines have this configuration. Counterfeit coins (fakes) and altered coins (real coins modified) are different things. According to the Ministry of Finance, the Mint identified eight counterfeit 500-yen coins in November 2003. This was the first time counterfeits of the new 500-yen coin, which began circulating in August 2000, were found. They were made of brass with nickel plating and are said to be unusable in most vending machines [4]. This is a separate case from altered coins made by shaving foreign coins. So, how are banknotes checked?
4. How do machines check banknotes using light and magnetic patterns?
Patents for banknote identification machines describe placing magnetic sensors and optical sensors on the transport path. They examine the magnetic pattern of the magnetic ink and the pattern of the amount of light passing through the banknote. They compare these to pre-stored standard data for genuine notes to determine authenticity [9]. One patent describes checking a coin’s material, thickness, and rim, while for banknotes, it checks "how magnetism and light pass through." Both compare against a standard for genuine items. The properties checked differ: coins are assessed by metal-related signals, while this patent checks magnetic and light patterns. This is one example from a patent and cannot be generalized to all current machines. If the method involves comparing against a standard, what happens to the machine when the standard for coins changes?
5. Why does the new 500-yen coin, which is the same size, affect vending machines?
The new 500-yen coin began issuance on November 1, 2021. It is made using a "bi-color clad" structure, where a clad disk of two metal sheets is fitted into a ring of a different metal, resulting in three types of material [6]. To prevent counterfeiting, "irregular oblique reeding" was adopted, where parts of the diagonal grooves on the side (four places: top, bottom, left, right) have a different shape. The Ministry of Finance states this is the world's first in regular currency [6]. The diameter is the same as the old 500-yen coin at 26.5 mm, and the weight is 7.10 g for the new one and 7.00 g for the old one [5]. The difference is 0.10 g, which is only about 1.4% (0.10 ÷ 7.00) of the old coin's weight. Vending machines recognize coins using sensors in the coin sorting device. Financial Field explains that older machines can accept new 500-yen coins if updated, but the spread of compatible machines is delayed due to costs, the global semiconductor shortage, and the shift to cashless payments [7]. Business Journal also explains that recognizing the new coin requires replacing the coin selector or refreshing each vending machine [8]. If the standard includes material and thickness, a coin with different contents will "not match the standard" even if the size is the same. However, this is an inference based on patents and papers, distinct from explanations by manufacturers. Can we verify this with our own hands?
6. How can you measure coins at home and calculate the numbers?
You can try one measurement and one calculation with coins at home. Measure the diameter with a ruler. Line up a 500-yen coin and a 100-yen coin and measure their diameters with a ruler. In the Ministry of Finance table, the 500-yen coin is 26.5 mm and the 100-yen coin is 22.6 mm [5]. Check if your measured values are close to the table values. You can also measure the 1-yen coin (20.0 mm) and the 50-yen coin (21.0 mm) to compare the diameter differences between coins. Look for pairs with similar sizes. In the table, the 5-yen coin is 22.0 mm, the 100-yen coin is 22.6 mm, and the 10-yen coin is 23.5 mm [5]. The difference in diameter is only 0.6 mm between the 5-yen and 100-yen coins, and 0.9 mm between the 100-yen and 10-yen coins. What would be difficult if you tried to tell them apart by diameter alone? Then, try explaining the concept of measuring material and thickness mentioned in this article in your own words. Calculate the weight difference. The difference between the new 500-yen coin (7.10 g) and the old 500-yen coin (7.00 g) is 0.10 g. If you calculate 0.10 ÷ 7.00 with a calculator, it is about 0.014, or about 1.4%. Thinking about why machine updates are discussed for such a small difference is one way to explore the topic. If you want to read the original sources, coin specifications are in the Ministry of Finance's "General Coins" table, and the description of the new 500-yen coin is on the Ministry of Finance's issuance page. The measurement methods for vending machines can be read in the text of patents JP3384803 and JP2002183794A.