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This little DIY kit is not only fun to build, but the result is also useful! A small calculator that allows you to do electronics calculations, among other things. An ideal DIY kit for the starter! |
Introduction to the CAI-150 electronic calculator kit
The end result
In the picture below, you can see the end result of a few hours of tinkering. A little calculator measuring 12.5 cm by 8.5 cm by 3.5 cm that lets you not only perform the primary arithmetic operations of addition, subtraction, multiplication and division, but also:
- draw square roots
- convert resistor colour codes to value and tolerance
- calculate series resistors for LEDs
- convert decimal numbers into hexadecimal numbers
- convert hexadecimal numbers into decimal numbers
The device is powered from two 3.0 V CR2023 button cells and features the familiar LCD display LCD1602 with two lines of 16 characters.
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| The end result of this little kit. (© AliExpress) |
Manufacturer, suppliers and price
It is not clear who the manufacturer of this calculator kit is. The kit is offered under various names via AliExpress and Amazon for rather different prices. A Google search for ‘electronic calculator kit’ yields a lot of hits. On AliExpress, the shops ‘LOTHYE Electron Store’, ‘OK GO Module Factory’ and ‘COME ON DIY Store’ offer the kit for a price of around € 10.00 including shipping.
Delivery of the DIY kit
The kit is delivered well-packaged within about a fortnight. What it contains is shown in the picture below. There are surprisingly few electronic parts present, most of the work is in preparing and soldering onto the PCB the twenty pushbutton switches that make up the keyboard. Note that two different enclosures are supplied. In our kit is an all-black enclosure consisting of just two parts. However, there are also kits in circulation where the case consists of a pair of transparent acrylic panels that you have to screw together.
NOTE: The two necessary CR2032 type button cells are not included!
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| The contents of the kit. (© 2024 Jos Verstraten) |
The manual
A miniature manual is indeed included. Experienced electronics hobbyists will have no problems tinkering together this kit using this piece of paper. Inexperienced solderers are better off using the very comprehensive eighteen-page step-by-step manual we found somewhere on the Internet and saved on our account at ‘archive.org’:
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| The mini manual supplied with the kit. (© 2024 Jos Verstraten) |
The circuit board
As with all cheap Chinese kits, there is nothing wrong with the PCB: double-sided, plated through holes, with solder mask and silkscreen. The two sides of the PCB are represented in the picture below.
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| The two sides of the PCB. (© 2024 Jos Verstraten) |
The technical specifications of this electronic calculator
- Power supply: 2 x 3.0 V CR2032 button cell battery (not included)
- Automatic switch-off: after ±15 seconds of inactivity
- PCB dimensions: 120 mm x 68 mm
- Housing dimensions: 125 mm x 85 mm x 35 mm
- Weight: 187 g
- Basic operations: + - x : √
- 4-ring or 5-ring resistance value and tolerance calculations
- LED current limit resistance calculation
- Decimal ↔ hexadecimal conversion
The electronics in this calculator
The figure below shows the full schematic of this little device. There is not much to tell about this. Pretty much everything is in the IAP15W413AS microcontroller from STMicroelectronics.
What is interesting is the power supply circuit. The two button cells are in series and the 6.0 V is reduced to 5.0 V by means of an HT7550-1 (U1). This stabiliser works internally with a MOSFET as a control transistor and needs only 100 mV voltage difference between the input and the output to work. However, the ‘OUT’ does not directly delivers the supply voltage for the electronics. That goes via an S8550 (Q1). When the circuit is at rest, the base cannot draw current and the transistor is in the off-state. The circuit receives no supply voltage and the calculator is off. When you press the ‘ON/AC’ button S4, the base is connected to ground via resistor R1, diode D2 and the switch. Transistor Q1 starts conducting and the +5.0 V supply voltage goes to the circuit. The microcontroller's ‘RX’ is grounded via diode D1 and switch S4. After releasing this switch, the microcontroller starts up. ‘SW’ is driven high so that the transistor Q2 starts conducting, takes over the operation of the switch S4 and keeps Q1 in conduction. The supply voltage remains available to the electronics.
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| The full circuit diagram of this calculator. (© IC Station) |
Building this electronic calculator
The parts list
The list below summarises once again all the components that should be in the package.
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| The complete parts list of this kit. (© IC Station) |
Soldering the PCB
With the help of the clear step-by-step guide that you can download from this page, soldering the PCB will be no problem even for an inexperienced beginner. For the sake of completeness, we show below a clear component layout of the part of the PCB where the electronics are.
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| The component layout of the electronics on the PCB. (© IC Station) |
Note the position of the black cathode ring on the two diodes D1 and D2. Do not swap the two transistors Q1 and Q2, in our package the type numbers were very difficult to decipher. Finally, pay attention to the correct position of the microcontroller U2 and make sure that all the pins are in the holes of the socket. The two PCB holes at P1 remain open. The purpose of this is that you can power the calculator from an external 6 Vdc supply if required.
The picture below shows the completely soldered PCB.
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| The completely soldered PCB. (© 2024 Jos Verstraten) |
Assembling the pushbutton switches
This is a very precise job! Two sets of the imprints of the twenty buttons are present on the manual provided.
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| The imprints of the keys. (© IC Station) |
The idea is to neatly cut out those paper squares and place them in the transparent caps of the keys. Afterwards, close those caps with the little blue parts (we can't think of a name for them) and then gently press them onto the pushbutton switches on the PCB.
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| One of the keys is equipped with its inscription. (© 2024 Jos Verstraten) |
Mounting the display on the PCB
Finally, solder the 16-pin PCB header onto the back of the display board. Afterwards, bend the long protruding pins at an angle of about 30 degrees. If you then push these pins into the connector on the PCB, the display board is at the correct angle for installation in the enclosure.
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| The mounting of the display on the PCB. (© IC Station) |
The completed PCB
The picture below shows the fully finished PCB, ready for mounting in the enclosure.
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| The PCB can be mounted in the enclosure! (© 2024 Jos Verstraten) |
Working with this electronic calculator
The standard operations
For the standard operations of addition, subtraction, multiplication and division, enter the commands as you write them down. So for the operation:
-8 + 2 = -6
you press consecutively:
- the ‘-’ key
- the ‘8’ key
- the ‘+’ key
- the ‘2’ key
- the ‘=’ key
Interestingly, the keys entered appear on the top line of the display and the result on the bottom line. A second striking point is that the numbers are limited to a maximum of seven digits, so the highest number you can enter is 9,999,999. Moreover, the result of a calculation may not exceed 10,000,000. A larger result results in the message ‘Can't calculate!’. Non-integer numbers are automatically rounded to three decimal digits.
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| Performing arithmetic operations. (© 2024 Jos Verstraten) |
Calculating the square root
This too is easy. First press the ‘√/⇕’ key, ‘√0’ appears on the top line of the display. Then enter the number whose square root you want to calculate. Press the ‘=’ key. When entering a negative number, the root sign automatically disappears. After all, in the real number world, negative numbers do not have a square root!
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| Calculating the square root. (© 2024 Jos Verstraten) |
Displaying the value and tolerance of resistors
Press the ‘MODE’ key until ‘Color Ring R C 5’ appears on the display. If you have resistors with only four colour rings, you must now briefly press the ‘√/⇕’ key. Then press the keys of the colour rings on the resistor from left to right. The value and tolerance will appear on the bottom line of the display.
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| Displaying the value and tolerance of resistors. (© 2024 Jos Verstraten) |
Calculating the series resistance of an LED
You should never connect an LED to a supply voltage without a series resistor! You can use this calculator to calculate the value of that series resistor. Press the ‘MODE’ key twice. Then enter the voltage difference between the LED's operating voltage and the circuit's supply voltage. Press the ‘√/⇕’ key, then enter the desired LED current in mA. Press the ‘=’ key. The value of the necessary series resistor appears on the display.
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| Calculating the value of an LED series resistor. (© 2024 Jos Verstraten) |
Converting from decimal numbers to hexadecimal and vice versa
Press the ‘MODE’ key three times. You can now enter the decimal number whose hexadecimal code you want to know. By pressing the ‘√/⇕’ key, you can convert from HEX to decimal. Here, the keys ‘.’, ‘=’, ‘+’, ‘-’, ‘x’ and ‘:’ serve to enter the HEX values ‘A’ to ‘F’.
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| Converting from decimal to hexadecimal and vice versa. (© 2024 Jos Verstraten) |
CAI-150 electronic calculator DIY kit
