CO2 Guard
A DIY Approach to Monitoring Air Quality

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Materials
Component list
Component List
Resistors
R1,R2,R3 = 330 Ω, 0.25 W, 5 %
R4 = 4.7 kΩ, 0.25 W, 5 %
R5 = 0 Ω, place wire, see text
R6 = 0 Ω, place wire for 5 V or leave open, see text
R7 = 1.5 kΩ, 0.25 W, 5 %
Capacitors
C1,C2 = 100 nF, 50 V, 10 %, X7R, lead spacing 5 mm
C3 = 100 µF, 25 V, optional , not mounted, see text
Semiconductors
D1 = BAT85, DO-35
T1 = BC327, PNP, TO-92
LED1 = LED red, 5 mm
LED2 = LED yellow, 5 mm
LED3 = LED green, 5 mm
Other
MOD1 = 1x10 SIL header, female, split in 1x4 and 1x5 (see Text)
MOD1 = MH-Z19C, version 400-5000 ppm, pins on bottom
MOD2 = Wemos D1 mini with 2x 1x8 SIL header
S1 = switch with built-in LED (momentary on, self-reset, blue circle)
Enclosure PP73BL, Supertronic
BZ1 = DC buzzer, 5 V, pitch 5 or 7.62 mm, diam. 14 mm max
heat shrink tube, I.D 2.4 mm, 20 cm
LED housing, for 5 mm LED (LED1..3), Black
FAN 5 VDC, 30x30x10.5 mm
12-pin male SIL header
PCB 210043-1 v2.0
Thin stranded wire to connect LEDs and switch:
stranded wire, 0.25 mm2, black (2x8.5+12 cm)
stranded wire, 0.25 mm2, red (8.5+12 cm)
stranded wire, 0.25 mm2, yellow (12 cm)
stranded wire, 0.25 mm2, green (12 cm)
stranded wire, 0.25 mm2, blue (8.5 cm)
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