Detson 121

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Calculators: Handheld: Detson 121

		Size (approx):	75mm x 138mm x 20mm (w x h x d) Weight 110g not including batteries.
		Power:	3.0V DC using 2 x AA size batteries. It accepts an adapter/charger through a socket on the top side to the far right. The Ch/On/Off switch allows you to use rechargeable batteries but there is a warning on the back stating “Do not use AC adaptor/charger when calculator is loaded with Alkaline battery [sic]”.
		Case:	The case is two-piece smooth and matt black plastic, brick-shaped with rounded edges. A small black printed metallic sticker with the brand sits above the display in its own recess. Another similar sticker sits on the rear with the model number. The green plastic display filter is flat and flush, giving a bright and easily viewed image. They keys are squishy and bouncy that on my example suffer from input bounce. The keyboard has a brushed aluminium panel printed with black lines for effect.
		Display:	8 digit green VFD with a ninth digit for negative and error indicators.
		Features:	Standard four functions with percentages, square root and four-function memory.
		Age:	1976
		Manufacturer:	Detson, but with no manufacturing details. Serial number 102248 is on the inside of the battery compartment.
		Comments:	This is one from a range of slim and functional calculators by Detson. It was obviously made as cheaply as possible. The logic, however, is sound being only let down by negative square roots. The original case is soft black plastic with an open top. Same case as the Detson 808.

Components:	1 x cpu; TexasTMS1042NL 7620 (week 20 of 1976), 28 pin DIL, 0.6" width 1 x 9 digit VFD display; single flat glass face 2 x transistors 3 x diodes 5 x capacitors 5 x resistors 1 x transformer: W523A
Boards:	The main cpu board (Bestek 12-0020) sits loosely above the keyboard assembly and is joined by a 17strong wires in the middle of the board! The latter is attached to the front with six screws.
Construction:	Remove the two screws from the back. Gently lever out the bottom as this is the side that appear to pop the easiest. The serial number label keeps the top two sides joined so carefully hinge off the back from the top.

Logic comments:	The (C) key is used once to cancel an entry whilst a second press clears the whole calculator
	Input overflow is suppressed inputting an ninth digit is ignored
	Negative numbers are shown with a "-” in the far left (ninth) digit, thereby allowing full eight digit negative numbers.
	There is automatic constant on all four functions
	Divide by zero shows the result zero and “C” in the far left (ninth) digit and is recoverable using (C)
	Overflow shows the result and “C” (or “E” if negative) in the far left (ninth) digit and is recoverable using (C)
	Negative square roots are allowed and result in a negative number
	There is indication of memory store; an upper “n” lights up in the far left (ninth) digit