Electronics Calculator App: 18 Tools and Worked Examples

Yes, there is an electronics calculator app for your phone that handles these jobs: the Tools screen in BlueGrays groups 18 tools into four sections: Resistors, Calculators, Capacitors and inductors and Reference. Seven of them are free (Color code, SMD code, Ohm's law, LED resistor, Capacitor code, Inductor code, LED voltages) and 11 are Premium. Nine of the Premium tools also open for free users as a Sample calculation: you see the tool working with ready-made values, and you need Premium to enter your own. In short, reading a resistor's value from its colors, finding how many ohms to put in front of an LED or working out a current with Ohm's law costs nothing, while design calculations such as the voltage divider, the 555, battery life and RC timing sit on the Premium side.

We have covered the theory of Ohm's law, series and parallel connections and capacitors in separate posts: Ohm's law, series and parallel resistor calculation and what is a capacitor. This post doesn't repeat the theory; it is about the tools themselves. First we look at how the screen is organized and at the full list of 18 tools, then we work through seven examples step by step, with the numbers the app will show you: color code, SMD code, LED resistor, voltage divider, 555 square wave, ESP32 battery life and RC time constant. At the end you'll find saved calculations, the pinout and transistor references, a lesson-by-lesson tool map and an "I have X, which tool do I open?" list.

Where is the Tools screen and how is it organized?

The Tools screen opens when you tap the Tools tab in the app's bottom bar. At the top is a search box reading Search tools: resistor, LED, 555…, below it a Recently used strip for the tools you have opened before, and then the tool cards in four sections. Premium tools carry a small Premium label on their card, so you can tell which ones are free without opening them. At the very bottom of the list there is a Missing a tool? card with a Suggest button. The order of the sections follows the questions you ask at the workbench: first identify the part in your hand, then calculate the circuit, and finally look something up. Here is what each section contains:

18 electronics tools: what they calculate and which are free

Example 1: reading a 220 Ω resistor with the color code tool

1. Open the Color code tool, stay on the Colors to value tab and choose 4 bands.
2. Hold the resistor with the gold band on the right and enter the colors from left to right: band 1 red (2), band 2 red (2), band 3 brown (multiplier ×10), band 4 gold (tolerance 5%).
3. The tool returns 22 × 10 = 220 Ω ±5% and also shows the tolerance range: 209–231 Ω.
4. On the same screen you see which standard series contain the value: 220 Ω is a very common value found in the E6, E12 and E24 series.
5. To be sure, measure the resistor on your multimeter's ohms range; a reading between 209 and 231 Ω confirms you read the colors correctly.

It also works the other way: type a value such as 220, 4k7, 4,7k or 1M2 into the Value to colors tab and the tool draws the bands. If you enter a value that can't be written with 4 bands, for example 4.99 kΩ, it shows the nearest value and tells you to pick 5 bands for the exact one. Five-band resistors have three digits, a multiplier and a tolerance band; a six-band resistor adds a temperature coefficient band at the end, which the tool shows in ppm/K. Colors can be misleading in dim light; brown and red, or orange and red, are often confused. That's why measuring with a multimeter is the most reliable check for a critical part.

Example 2: SMD codes 472, 1002 and 01C

Just type the code printed on the part into the SMD code tool, letters and digits exactly as they appear; the tool recognizes the format by itself and explains it under How to read it. The most common confusion is with EIA-96 codes such as 01C: the 01 here is not a value but a position number in the 1% E96 series. Value number 1 is 100 and the letter C means ×100, so the result is 10 kΩ. In other words, 1002 and 01C are the same resistor written in two different formats. Two more free tools work the same way for capacitors and inductors: Capacitor code decodes 104 as 10 × 10⁴ pF = 100 nF and reads the K in a code like 103K as a 10% tolerance; Inductor code gives 101 as 100 µH and 4R7 as 4.7 µH.

Example 3: a red LED at 5 V and 15 mA

1. Open the LED resistor tool and pick red as the LED color. The tool uses a typical forward voltage of about 1.85 V for a red LED.
2. Set Supply to 5 V (an Arduino Uno pin) and Current to 15 mA. You are using a single LED, so leave LEDs in series at 1.
3. The tool works out the voltage left for the resistor: 5 − 1.85 = 3.15 V. The calculated resistance is 3.15 V / 15 mA = 210 Ω.
4. 210 Ω is not a value you can buy, so the tool suggests the next standard value up: 220 Ω. That's the red-red-brown resistor from the first example, and the tool draws its bands too.
5. With 220 Ω the actual current is 3.15 / 220 ≈ 14.3 mA. If the LED voltage is at its upper limit (2.5 V for red), the current drops to about 11.4 mA; the LED still lights up nicely.
6. The power dissipated in the resistor is about 45 mW. The tool recommends at least 1/8 W with a twofold margin; the ordinary 1/4 W resistor in your kit is more than enough.

Example 4: a ready-made resistor pair to drop 5 V to 3.3 V

Say you want to connect the output of a 5 V sensor to the 3.3 V input of an ESP32 or a Pico. The Voltage divider tool has two modes: in Find output you enter R1 and R2 and the tool gives the output, current, ratio and total power; in Find resistors you enter the input and the target output and the tool suggests Ready-made pairs of standard resistors. The sample calculation opens with a 5 V input, 10 kΩ on top and 20 kΩ at the bottom: the output is 5 × 20 / 30 = 3.33 V and the divider draws about 0.17 mA. In Find resistors mode, when you enter 3.3 as the target and switch between the E12, E24 and E96 series, the suggestions change; here are the results for the first two series:

Tap a pair and its values move into the calculation, where you see the current and power. When choosing between pairs with the same ratio, look at the current: a 4.7 kΩ / 9.1 kΩ divider draws about 0.36 mA from 5 V, while 47 kΩ / 91 kΩ draws 0.036 mA. Small resistors waste current, and very large ones make the analog reading more sensitive. That's why the app's note tells you to keep R1 + R2 usually between 10 and 100 kΩ, and reminds you that a divider output is not a power supply: connect a motor, relay or LED and the voltage drops. A divider is for ADC inputs and for shifting 5 V down to 3.3 V. To see the same idea in practice, look at the free Measuring Light Intensity with an LDR project: the LDR and a fixed resistor form a divider there too, and the Arduino reads the voltage in the middle.

Example 5: a 1 Hz square wave with the 555

The 555 timer tool has Square wave and Single pulse modes. For a square wave you enter three values: R1 (between pin 7 and VCC), R2 (between pin 6 and pin 7) and C (between pin 6 and GND). The sample calculation opens with R1 = 1 kΩ, R2 = 68 kΩ and C = 10 µF. The frequency is f ≈ 1.44 / ((R1 + 2·R2) · C): 1.44 / (137 kΩ × 10 µF) = 1.44 / 1.37 s ≈ 1.05 Hz. The tool also shows the times separately: HIGH time 0.693 × (R1 + R2) × C ≈ 0.478 s, LOW time 0.693 × R2 × C ≈ 0.471 s, period ≈ 0.95 s. The duty cycle is the HIGH time divided by the period: 0.478 / 0.95 ≈ 50.4%. The LED preview on screen really blinks at this speed; as the frequency rises, the tool notes that the eye can no longer see the blinking and the LED looks as bright as the duty cycle. A faster example: with R1 = 1 kΩ, R2 = 10 kΩ and C = 100 nF the tool shows about 687 Hz and a 52.4% duty cycle, an audible tone on a buzzer. In Single pulse mode the length is t = 1.1 × R × C: with 100 kΩ and 10 µF that is 1.1 seconds. For the IC's pins and how to build the circuit, see the 555 timer IC post.

Example 6: ESP32 battery life with deep sleep

1. In the Battery life tool, pick an 18650 as the battery (2600 mAh). The presets also include CR2032, 9 V, LiPo and 2 × AA, and you can type the capacity by hand.
2. Choose ESP32 · deep sleep as the current profile: the board stays awake for 5 seconds drawing 80 mA, draws 10 µA in sleep, and the cycle repeats every 600 seconds (10 minutes).
3. Work out the average current: (80 mA × 5 s + 0.010 mA × 595 s) / 600 s = (400 + 5.95) / 600 ≈ 0.677 mA.
4. Find the usable capacity: the tool counts 85% of the capacity to allow for battery aging, cold weather and regulator loss: 2600 × 0.85 = 2210 mAh.
5. Find the runtime: 2210 / 0.677 ≈ 3266 hours, or about 136 days (4.5 months).

The lesson of the table is clear: in the preset, about 98.5% of the energy is used while awake, and the tool says so plainly: "Most of the energy is used while awake". Cutting the sleep current from 10 µA to 5 µA barely changes the result, while cutting the awake time from 5 seconds to 1 second makes the battery last almost five times longer. The tool draws the current profile on a two-cycle logarithmic chart, so you see the short but high wake-up peak and the long but low sleep floor at the same time. One caution: boards like the Arduino Uno draw milliamps even in sleep because of their regulator and power LED, so putting an Uno to sleep doesn't stretch battery life the way it does on an ESP32. The Premium ESP32 Deep Sleep — Battery Saving (OLED + RTC RAM Counter) project in the app shows this scenario in code; for ways to power a board from batteries, see running an Arduino on batteries.

Example 7: RC time constant and cutoff frequency

In the RC time constant tool you enter a resistor and a capacitor and choose the Charge or Discharge curve. For the 10 kΩ and 100 µF of the sample calculation, τ = R × C = 10,000 Ω × 0.0001 F = 1 second. The tool builds a table: at 1τ (1 s) the capacitor reaches 63% of the supply voltage, at 2τ 86%, at 3τ 95%; at 5τ (5 s) it counts as full at 99.3%. The time to reach 50% is 0.69 seconds. Used as a low-pass filter, the same RC has a cutoff frequency of fc = 1 / (2π × R × C) ≈ 0.16 Hz. Change the capacitor to 100 nF and τ drops to 1 millisecond with a cutoff of about 159 Hz, a more useful range for smoothing a PWM output or a bouncing button. You can type values like 100n, 4u7 or 22p; if the capacitor has a code such as 104 printed on it, decode it first with the Capacitor code tool. To reduce several parallel capacitors to a single value, use the Series / parallel C tool.

Saved calculations, Recently used, search and Suggest a tool

• Saved calculations (Premium): the Save button below a tool stores the calculation with its inputs in your account; it isn't lost if you change phones and shows up on all your devices. Tap a saved entry and the tool reopens with the same values; remove the ones you don't need with Delete. With a free account, tapping Save opens the Premium overview.
• Share: sends the result as a short text by message or through another app, a quick way to pass the resistor you calculated to your lab partner.
• Recently used: the last six tools you opened sit in a strip at the top of the screen, so you get back to your favorite tool in one tap.
• Search: typing into the Search tools: resistor, LED, 555… box filters the tools and shows how many results were found. If nothing matches, the screen points you to the suggestion card.
• Suggest a tool: tap Suggest on the Missing a tool? card at the end of the list and briefly write which calculator or table would help you. The examples the app gives are "7-segment display pins" and "buck converter calculator".

Pinout and Transistors: reference tables for the workbench

The Pinout tool shows a board's pins on a drawing of the board. Besides the Arduino Uno, ESP32 and Raspberry Pi Pico, the list includes boards from the same families: Arduino Nano, Mega and Leonardo, ESP32-S3 and C3 SuperMini, Raspberry Pi 4 and Zero 2 W. The filters at the top highlight Analog, PWM, I2C, SPI, UART, DAC and Caution pins one group at a time. Tap a pin and the SELECTED PIN card opens with its logic voltage, its current limit (20 mA recommended on the Uno, 40 mA at most; 4 mA by default on the Pico, up to 12 mA) and an explanation of what that function is for; you can also copy sample code for the pin. The notes prevent practical mistakes: on the ESP32, ADC2 pins can't do analog reads while Wi-Fi is on; on the Pico, analog input only works on GP26, GP27 and GP28; and a pin used for I2C can't do analog reads at the same time. For every Uno pin there is also the Arduino Uno pinout post.

The Transistors tool lists the most widely used NPN and PNP transistors and MOSFETs; type BC547, TIP120 or IRLZ44N into the search box. Each card shows the pin order (printed face toward you, legs down, left to right), the voltage and current limits and an Equivalent line. That line points to an important trap: the BC547 and 2N3904 can replace each other, but their pin orders differ (CBE on the BC547, EBC on the 2N3904); swap the part without checking the circuit and the transistor ends up reversed. The two other tools in the Reference section are useful in class too: in Logic gates you try AND, OR, NOT, NAND, NOR, XOR and XNOR by tapping the A and B inputs and see the truth table and IC numbers (7400 and 4011 for NAND, for example). The free LED voltages table lists the typical and maximum voltage and the maximum current for each color. For the theory, see what is a transistor and logic gates.

For students and teachers: which tool for which lesson?

The tools don't replace a textbook, but they are a good second step for checking a calculation done by hand. Say you are teaching Ohm's law to a class of 24: students do the calculation on paper first, then compare it with the free tool, and finally measure with a multimeter. For a lesson plan that builds this three-step flow in detail, see the Ohm's law activity post. For the tools marked "sample" in the table, a free account only shows the ready-made example; if students need to try different values, either project a Premium account on the board or build the task around the sample values.

Tool picker: I have X, which tool do I open?

• A resistor with colored bands → Color code.
• A tiny SMD resistor with a 3–4 character code → SMD code.
• A capacitor marked 104, 473 or 2n2 → Capacitor code; combining several → Series / parallel C.
• An inductor marked 101 or 4R7 → Inductor code.
• An LED and a supply voltage → LED resistor; wondering how many volts an LED color needs → LED voltages.
• A calculated value you can't buy, for example 5 kΩ → Standard value: in E12, 4.7 kΩ + 330 Ω in series (5.03 kΩ) or 10 kΩ ∥ 10 kΩ in parallel (exactly 5 kΩ).
• Two known quantities, for example voltage and resistance → Ohm's law; a chain of several resistors → Series / parallel.
• A 5 V signal and a 3.3 V board → Voltage divider.
• A 555 IC and a few resistors and capacitors → 555 timer; the charge time of a single capacitor → RC time constant.
• A battery-powered ESP32 project → Battery life.
• A weak sensor signal that needs amplifying → Op-amp gain (the theory is in what is an op-amp).
• A transistor you don't recognize → Transistors; a board where you don't know what goes on which pin → Pinout.
• A logic IC such as the 7400 → Logic gates.

To sum up: everyday calculations such as reading colors and codes, Ohm's law and LED resistors are ready on your phone with the free tools; design calculations such as the voltage divider, the 555, battery life and RC timing can be explored in the sample first and, if you need them, done with your own values with Premium. A calculation alone doesn't make a circuit work; you still have to try the result on a breadboard. Open the Tools screen in the BlueGrays app, repeat one of the examples in this post with your own parts, then build the free LED Blink or Traffic Light project with the same resistors and LEDs. If you want to control your circuit from your phone, try BlueGrays IoT under Workshop › My devices › Add new device. You pick one of 8 ready templates for an ESP32 or Raspberry Pi Pico W. Ohmi prepares the code with your device key, and you upload it once from Arduino IDE with the BlueGraysIoT library. Your first device is free: 1 device, a chart for 1 value and 24 hours of history. For the steps, see the guide to controlling an ESP32 from your phone.