With recent advances in semiconductor technology, the maximumluminous intensity of red l.e.d.s has increased to several candelas. Now consider that human flesh is translucent to red light. Combine these two facts, and we have the basis for the present project.
The average cheek has a thickness between one and two centimetres, and this makes it possible to “backlight” a cheek from the gums with a high intensity l.e.d., causing it to glow in the subdued light of a Christmas party or disco etc. The slimline circuit featured here is slipped between the gums and cheek, and is activated by the presence of saliva. Afterwards, when it is removed and cleaned-up, its current consumption drops to practically zero. Therefore, the circuit could likely be used for years in a row without battery replacement.
The full circuit diagram for the Christmas Cheeks is shown in Fig.1. It incorporates two main building blocks – a solid-state switch, and a timer for pulsing l.e.d. D1. Since it would be difficult to include an on-off switch in any circuit which is exposed to saliva, a solid-state switch in the form of transistor TR1 is employed. This switches on the circuit only when it is placed in the mouth. About one-third of a milliamp flows across the “switch”, and this may be experienced as a slightly metallic taste on the tongue. This flows across the baseemitter junction of TR1, causing TR1 to conduct. With the choice of a BC107B transistor as the solid-state switch, collector current is limited to roughly 16mA. This limits total power consumption to about 50mW, which suits the maximum output of the astable oscillator IC1, as well as the maximum power dissipation rating of modern high intensity l.e.d.s. The l.e.d. D1 should have a minimum power dissipation rating of 50mW – alternatively, replace IC1 with the bipolar NE555N i.c., in which case a 30mW rating will be adequate. Note that not all l.e.d.s live up to their luminous intensity ratings, and it may be best to test D1 before employing it. Capacitor C1 charges through resistors R1 and R2, and discharges through R1 alone. With equal values for R1 and R2, this provides a duty cycle of about onethird for l.e.d. D1, with the rising and falling voltage at the trigger and threshold inputs (pin 2 and pin 6) of IC1 causing output pin 3 to switch either “high” or “low”. Thus, l.e.d. D1 is “on” for about 1·5 seconds, and “off” for about 3 seconds. For better control of power through transistor TR1 (to prevent power surges in particular), no supply decoupling capacitor is used. The specified lithium battery is rated 190mAh, which ensures a continuous life of about 12 hours when inserted in the cheek .
Cover the Circuit
The completed circuit board, including battery, now needs to be encapsulated in polyester resin. The author dipped the p.c.b. in polyester resin, then wrapped it in tissue paper, tied it with thread, and dipped it a second time in polyester resin, see photograph. Since polyester resin has very low conductivity, this will not draw any significant current from the battery when wet.Check for a conductivity of 1M or greater before using any other sealant. When dipping the p.c.b. in the resin, hold the board at the corner marked A in Fig.2, and submerge all except this corner in the resin. This corner may also be covered with sticky tape (Sellotape) . Only the two broad copper tracks at point A should be left exposed, these being the contact “switch” pads between the positive supply and base connection of TR1, the solid-state switch.
Testing and Usage
For initial testing, moisten a finger, and hold it across the solid-state switch’s contacts (and across the contacts only). Do not short-circuit the contacts, since this could destroy the circuit. After about three seconds, l.e.d. D1 should illuminate, then it should pulse at a slow rate, as indicated earlier. Once the circuit is encased in polyester resin, insert it between gums and cheek, with the battery resting between the bottom gum and cheek. The l.e.d. should rest against the upper gum, facing the cheek. For added effect, a second unit may be built for the opposite cheek. If desired, a simple brace may be attached to the p.c.b., to make it more stable in the mouth. The dimensions of Christmas Cheeks are intended for an adult mouth only. Certainly, the circuit is NOT recommended for youngsters below 8 years of age, since this may represent a choking hazard. For a smaller mouth, the circuit may need to be mounted horizontally in the cheek for a comfortable fit.