The reflex, or “red-dot,” sight is a popular category of optical aiming/pointing aids, which finds diverse applications in astronomy, archery, shooting, etc. In the reflex-style sight, a source —typically a high-intensity LED — is reflected from a curved, transparent optical (reflex) element through which you view the target. The image of the source (the red dot) thus appears superimposed on the target image, indicating the point of aim. The reflex sighting system offers several advantages over telescopic and open sights, including rapid and intuitive target acquisition, and a wide field of view (FOV).
For best performance, the intensity of the sight’s light source must at least roughly match the illumination level of the target. If the source is too dim, the aim-point dot loses itself in the brightness of the target, while if too bright, the dot flares, obscuring the point of aim and making precise pointing difficult. Although manual adjustment of dot intensity works, it detracts from the rapid and intuitive operation of the sight, making automatic adjustment a highly-preferred capability. This auto-adjust feature was demonstrated in a Design Idea first published in EDN nearly 19 years ago, but there was room for some improvements.
To appreciate those improvements, let’s first look at the original circuit, shown in Figure 1. Phototransistor Q1 senses the target brightness level and uses it to automatically adjust the reflex LED current and intensity, while maintaining a constant dot size over a wide range of ambient-light levels. Potentiometer R1 splits Q1‘s photocurrent, IP, between the LED driver Q2, and bias transistor Q3 (connected as a diode). R1‘s value is set at the factory as a one-time calibration of the ratio between drive current, IL, and ambient light intensity. Since the light shield is co-aligned with the sight and it roughly mimics its FOV, Q1 receives a representative average of the target illumination level. As a result, a one-time calibration of R1 assures that the dot’s intensity will remain relatively constant across a wide range of incident light.
The original design has been in useful service since its development 20 years ago. But of course few old lilies can’t benefit from a bit of new gilding.
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One shortcoming of the original design is its need for an opaque lens cover to be placed over the light input port when not in use. This is necessary to avoid premature battery demise if the sight is stored in a brightly-lit environment. Of course a simple manual ON/OFF switch could be added, but a manual switch only works if you remember to use it, and it’s easy to forget. The improved circuit shown in Figure 2 provides an automatic solution by adding a CMOS power-off timer that disconnects the battery after an hour of inactivity.
The timer function is based on the venerable (okay, ancient) metal-gate CMOS 4060B 14-bit oscillator/divider. The ’60s input stages form a gated oscillator, in this case with a period of ~0.44 sec set by the C2, D1, R4-6 network. The output goes to the device’s internal 14-bit binary counter, which multiplies the oscillator period by 8192 to yield the nominal 3600 sec = 1 hr power-on interval. Power-on begins with reset of the 4060B by pulling pin 12 high. This drives pin 3 (213) low, turning on Q4 (which supplies power to the optical circuits) and enabling the oscillator. After 8192 oscillator periods (1 hr), pin 3 returns high, turning off Q4 and the oscillator.
The new design features another, somewhat novel, improvement: a skin-resistance touch plate that serves as the interval-starting reset switch instead of a conventional pushbutton. The strips of copper tape that serve as the touch plate are less expensive than the pushbutton and provide a simplified, highly-ergonomic user interface. In addition, they can be easily integrated with a bow, gun stock, or other sighted device where mounting a mechanical switch would be more difficult or unsightly (pun intended).