🌆 Curfew Beeper

What you're building

Set it on the windowsill in the morning. It sits silent all day. The sun goes down. Beep. Time to come inside. That's it — a little curfew clock that runs on daylight.

By now you own all four ways to mix a sensor brain with an output mouth from what's already in your box. This project is the last of the four. Look at the grid:

	LED output	Buzzer output
Dark triggers	Automatic Night Light	Curfew Beeper (this)
Bright triggers	Sunrise Alarm	Cookie Jar Alarm

Every column is "same brain, different mouth." Every row is "same mouth, different brain." Four projects from two brains × two mouths. A small parts box can do a lot when you recombine the same pieces. This project finishes that set.

The big idea: same brain as Automatic Night Light

The sensing half is identical to the Automatic Night Light. Divider with 10 kΩ on top and the LDR on the bottom. When the room is bright, the LDR shrinks to about 1 kΩ, and the middle point sits low; when the room goes dark, the LDR balloons past 100 kΩ and the middle rushes up past the transistor's wake-up line.

• Bright room, LDR ≈ 1 kΩ: V_middle = 4.5 × (1 ÷ 11) ≈ 0.41 V. Well below the 0.7 V wake-up line. Transistor off, buzzer silent.
• Dark room, LDR ≈ 100 kΩ: V_middle tries to climb toward 4.5 × (100 ÷ 110) ≈ 4.1 V. The base clamps it at 0.7 V and drinks the extra push as current. Transistor on, hard.

If any of that is unfamiliar, go back to Automatic Night Light. This uses the same math, the same divider, and the same reasoning.

The new part: same mouth as Cookie Jar Alarm

Now the output. You've done the LED lane many times. Here you're using the buzzer lane, exactly as it lived in Cookie Jar Alarm — and for the same reason: the active buzzer already limits its own current at about 150 Ω-equivalent, so no 220 Ω is needed on this lane. The buzzer just hangs from the top rail down into the transistor's collector.

When the transistor is fully on, the buzzer sees 4.5 − 0.2 = 4.3 V (that 0.2 V is the transistor's saturation drop, V_CE(sat)). By Ohm's law:

I_buzzer = 4.3 V ÷ 150 Ω ≈ 29 mA

Comfortably inside the buzzer's 3–5 V happy zone, so it sounds normal — same volume as Door Buzzer and Cookie Jar Alarm.

Do the math (all together now)

For the dark-room case, working it top to bottom:

• Divider push after the base clamps: V_middle ≈ 1.0 V (the base drinks enough current to hold the middle just above 0.7 V — a classic saturation signature)
• Base current: (1.0 − 0.7) ÷ 1 kΩ = 0.3 mA
• With β ≈ 200, the transistor could push up to 60 mA through the collector. The buzzer only asks for ~29 mA. That's plenty of headroom, so the transistor saturates and the buzzer runs at full tone.

For the bright-room case:

• V_middle ≈ 0.4 V, below the 0.7 V wake-up
• Base current: essentially zero (a few µA of leakage)
• Buzzer current: essentially zero (the transistor is open like a broken switch)

Battery drain when it's quiet: microamps. You could leave this on the windowsill all week and barely dent the AAs — the alarm only sips power when it's actually firing.

Build it

If your Automatic Night Light is still standing on the breadboard, this is a 30-second swap: pull out the 220 Ω and the LED, drop the buzzer between the top rail and the transistor's collector, done.

Fresh build:

1. Power rails: battery + to a top row, battery − to a bottom row.
2. The sensing half (LDR on bottom): battery + → 10 kΩ → middle row → LDR → battery −.
3. The base link: middle row → 1 kΩ → transistor Base (middle leg with the flat face toward you).
4. The output half: battery + → buzzer + leg; buzzer − leg → transistor Collector (right leg). Emitter (left leg) → battery −.
5. Point it at a lamp: silence. Cup your hands over the LDR: after a heartbeat, beeeep.

You'll hear the buzzer fade in as your hand closes over the sensor — same in-between region the Automatic Night Light introduced. Between transistor-off and transistor-fully-on, there's a mushy analog zone where the buzzer scratches before it commits to a tone. Level 6's 555-timer chips will turn that mush into a crisp snap; for now, hearing the transition is part of the lesson.

Deploy your curfew clock

• Put the whole breadboard on a west-facing windowsill (or east, if you want an alarm clock instead of a curfew). The LDR needs a clear view of the sky.
• Do a daytime test with the room lights off and the curtains drawn to simulate dusk — if it beeps in that state, the trigger is where you want it. Then leave it running and listen for the real sunset.
• If the buzzer chatters on-and-off around dusk (mushy transition), that's the analog zone above. It'll settle into steady beeping within a few minutes as the sky darkens more.

Where the "brain × mouth" grid leads next

At Level 4 you'll add a motor. That's a third mouth for the same sensing brains you've already built. Automatic Night Light brain + motor = a fan that spins when the room goes dark. Cookie Jar Alarm brain + motor = a paper barrier that slams down when the lid opens. Six projects from two brains × three mouths. Then Level 7's Arduino replaces the transistor + resistor + divider altogether. You still use the brain-vs-mouth split to think about the circuit.