YRabbit's notes - Electronics

Programmer's Calculator

2025-10-19T16:07:00+10:00

Finished look

Programmer's calculator (buzzer)

2025-05-01T21:46:00+10:00

A new peripheral for the calculator - a passive buzzer.

Calculation

Buzzer connection diagram

A passive buzzer with a coil resistance of 16 from an Arduino kit is used. Here’s how it looks with a soldered diode. The diode is necessary because this is an electromagnetic buzzer, not a piezo one - though even piezo buzzers might need one since they generate electricity when deformed (think of piezo lighters).

Close-up of the buzzer with a soldered diode

Let’s take a look at the buzzer’s specifications from TZT:

First part of the buzzer description

Buzzer parameter table

Determining the value of . The allowable current is , Assuming the average voltage across the buzzer coil is zero, we get

Let’s check the parameters of the 2N3904 transistor:

2N3904 transistor parameters

Assuming , we calculate:

Since the collector current is already limited, it’s fine even if we take the minimum . Thus, the base current .

From this, we derive:

As for , it's pull-down resistor with a sufficiently high value. Alternatively, such a resistor could simply be enabled on the CH32V003 pin.

Programmer's calculator

2023-04-15T15:15:00+10:00

This is a very simple thing - a desktop calculator for operations in different number systems.

It is imagined as having a few lines of display, a lithium battery, and a keyboard with a small number of keys.

Keyboard

I will use Rue Mohr's scanless keyboard design http://ruemohr.org/~ircjunk/tutorials/elex/scanlessKeyboard/scanlessKeyboard.html .

Here is draft of the schematics:

The calculator keyboard diagram

I use the highlighted fragment to determine the resistor ratings.

Fragment of the keypad circuit.

Let me speculate: the CE current of transistor Q1 is determined by the resistance of the 45k pull-down resistor of the controller pin (not shown in the schematic):

Based on the datasheet chart, I determine that to go into saturation mode, it is enough to provide 0.6V voltage at the base. The same applies to transistor Q2.

Graph from the 2n2222 datasheet

Graph from the 2n2907 datasheet

Hence the voltage drop across resistor R1:

As can be seen from the same graphs, the base-emitter saturation current is 10 times less than the :

I choose the resistor R2 arbitrarily 10k - anyway the voltage drop on it equals .

You cannot move higher because of the danger of getting too little current, on the other hand a high current will have no effect. So I choose the value of 20k.

The current through resistor R1 is actually the same as predicted

current

Rue Mohr's <https://twitter.com/RueNahcMohr> told me to change the circuit so as to get a "keystroke detection circuit":

Modification of the keyboard circuit