Compact Egg Candler Circuit

An egg candler is a simple device used for testing incubating eggs to determine if they are viable or not. The device provides a light source and channels that light into the egg so you can determine if the egg is developing into a chick. The egg candler presented here is designed to be handheld but also can be left on a work surface. Rechargeable-battery-powered for convenience, it uses rich-output, high-efficiency white LED illumination so there are no incandescent bulbs to replace and no fears about overheating the eggs during inspection.

Circuit Description

The circuit has been kept very simple in the interest of reliability. The power supply for the circuit is provided by a 3.7-V Lithium-ion battery (BAT). The output of this battery is connected to a 5-mm, round, high-flux white LED (LED2) through a current clipper resistor (R2). A precision battery charger is also included for use with the battery. The charger circuitry, based on MAX1879 (IC1), is suitable for use with any standard 5-V USB power supply/smart phone travel charger. The MAX1879 single-cell lithium-ion battery charger chip (available in 8-pin μMAX package) utilizes an efficient pulse-charging architecture, which combines the efficiency of switch-mode chargers with the simplicity and low cost of linear chargers. The red LED (LED1) connected directly to pin 3 of MAX1879 works as a visible charge-status indicator. The FDC638P — PMOS FET (T1) in the circuit is either on or off, allowing the power source to be directly connected to the Li+ battery or completely disconnected. One reason the circuitry is so simple is because the charging current is set by the external (current-limited) power source, not by the MAX1879 chip!

egg candler circuit

MAX1879 datasheet

Circuit Construction

Construction of the circuit is relatively simple and can be made using a small piece of universal SMD prototyping board. If you design your own PCB, to obtain optimum performance, adhere to the advice given in the MAX1879 data sheet. This is also very helpful when it comes to using the circuit for real-world applications.


For the sake of neatness, the finished circuit can be mounted in a plastic/acrylic casing to fit comfortably in the hand.


(making of the egg candler-from author’s lab)

How To Use the Egg Candler?

Start candling your eggs on Day 4 of the incubation process using the following steps:

  • Remove eggs from incubator and place in empty egg cartons.
  • Dim the lights in the room.
  • Switch your egg candler on.
  • Place the large end of each egg (one at a time) at the base of the candler.
  • Observe egg to determine if it is fertile (good) or infertile (bad). Refer to the image shown below.
  • Discard infertile eggs and return the fertile eggs to the incubator (within 30 minutes).

good bad egg

If you are unsure about the fertility of an egg, mark it with a question mark in pencil and observe it later.

Component List

  • R1 = 10K (SMD 0805)
  • R2 = 4R7 (SMD 0805)


  • C1 = 1nF (SMD 0805)
  • C2 = 2u2 (SMD 0805)


  • LED1 = Red (5mm, thru-hole)
  • LED2 = White (5mm, high-flux, thru-hole)
  • D1 = SS14 (DO-214AC)
  • T1 = FDC638P (Super SOT –6)
  • IC1 = MAX1879 (8-pin μMAX)


  • BAT = 3.7-V Lithium-ion battery
  • S1 = SPST Switch
  • J1 = USB (Female)

(prototyped with components collected from